DETHEROC

The dreadlord of the Nathrezim, Detheroc, and his two brothers, Balnazzar and Varimathras, were left to watch over the Legion's holdings in Lordaeron when Archimonde and the main force of the Legion left for Kalimdor to deal with the fleeing orcs and humans, as well as the night elves who had defeated them in ages past.
In the aftermath of the Third War Detheroc began to notice that much time had passed without hearing a word from Archimonde. One day, Arthas stormed into the palace and informed them that their Legion was defeated, and that their deaths would seal that fate. They were able to flee before he could strike. Hiding in Tirisfal Glades, they coerced Sylvanas into assisting them, and planned a coup to take back Lordaeron and rule the newly coined Plaguelands. Detheroc and his brothers assumed control of the Capital City and ambushed Arthas, but the resilient Death Knight battled his way through their forces and fled across the sea, to his master in Northrend.

With Arthas gone, Detheroc and the other Dreadlords could concentrate their power in Lordaeron. When Varimathras asked Sylvanas to join their new order, she refused, butchered his forces, and turned him to her own side. Detheroc was not so easily defeated. He used his powers of domination to control the human general, Garithos, and when Sylvanas launched her next attack upon him, he turned Garithos and his forces loose on her. Now with Varimathras at her side, Sylvanas managed to keep Garithos at bay as she stormed into Detheroc's base and "tore him to bloody shreds" as Sylvanas put it.

ANETHERON

A cunning and powerful dreadlord, Anetheron was second only to Tichondrius himself (he later took the place of Tichondrius for a brief period) in the ranks of the nathrezim. Anetheron waited in the Twisting Nether, acting as emissary of Archimonde to Tichondrius. He was one of the dreadlords assigned to watch over the Lich King, and grew agitated when they began to suspect the Lich King of treachery and possibly becoming a threat to the Burning Legion. But Tichondrius was confident that, despite his obvious ulterior motives, the Lich King was still providing the tools they needed to make way for the invasion. In a meeting in the Twisting Nether, Tichondrius assured Anetheron that everything was still well in his control.

When Archimonde came to the world, Anetheron went with him, and served as a personal guardsman. Anetheron showed his tenacity when he led the Scourge against the Great Alliance consisting of the Sentinels, Alliance, and Horde in the final, climactic Battle of Mount Hyjal. He invaded the Great Alliance's base several times during the battle, calling infernals to serve him, with the objective of defeating the resistance so that Archimonde could ascend to crush the World Tree. However, Anetheron was killed during the battle

After his death, it is presumed that Mephistroth acts as the new leader of the nathrezim.

KAZZAK THE SUPREME

Kazzak the Supreme is a powerful elite doomguard commander. Once commanding the forces of the Burning Legion on the world of Azeroth, Kazzak was responsible for the reopening of the Dark Portal, and has since relocated to Outland, where he commands the Legion's armies there against all who would oppose them. Also known as Kil'jaeden's Right Hand and Lord of the Doomguard demons, Kazzak was one of Archimonde's most trusted lieutenants during the Third War.

Lord Kazzak's main goal — indeed, his only goal — was to reopen the Dark Portal and return to Draenor, now the shattered realm of Outland. It was unknown if he had the intent to serve Illidan or raise an army to fight the night elf betrayer. Two years after the founding of Durotar, with the future in peril, a relic of the past had also surfaced, radiating renewed energy. This relic had enabled Lord Kazzak to activate the current Dark Portal, thereby reopening the gateway to the shattered realm of Outland. With the portal now opened, Lord Kazzak was seen departing through the Dark Portal. In his stead he left his minion, Highlord Kruul, to sow fear and confusion among the defenders. Accompanied by a host of infernals, Kruul was seen near a number of major cities in Azeroth. Once Lord Kazzak passed through back to Outland, he was promoted to Doom Lord Kazzak and set up his throne on top of Throne of Kil'jaeden, a mountain in the Hellfire Peninsula, which he protects to this day.

Unlike many demon leaders, who tend to stay behind the lines and direct their minions against a foe, Lord Kazzak loves nothing more than to wade right into the heat of battle. There, he shatters formations with his shadow storm ability while laying waste to individual heroes with his mighty greatsword. As the battle enters its most critical phase, he flies into a rage, reveling in an orgy of destruction.

HAKKAR THE HOUNDMASTER

One of the primary lieutenants of the Burning Legion during the time of the War of the Ancients, Hakkar the Houndmaster was the first member of the Legion to set foot upon Azeroth. He was sent through the portal created from the Well of Eternity to assist Xavius in strengthening the portal, preparing for the rest of the Burning Legion to come forth. Hakkar's primary weapon was a whip which, besides traditional use, could be used to summon a seemingly limitless supply of felhounds to do Hakkar's bidding. Hakkar attempted to remove Krasus and Malfurion from the resistance, but quick intervention by the Sisters of Elune saved the wayward pair. Hakkar inflicted fatal damage upon the High Priestess of Suramar and was about to murder Tyrande when Malfurion Stormrage stepped in to prevent his love from being destroyed, calling down a fierce lightning storm to vanquish the Houndmaster. Baelmon the Hound-Master is presumably his successor. He is described as a huge skeletal knight, with a horned skull.
In the original timeline, Hakkar survived the War of the Ancients and was killed during the Third War. When the Old Gods warped time and space in an effort to escape their prison, the timeline was altered in a variety of ways, among them Nozdormu's efforts to stop them, which in turn altered the course (though not the outcome) of the War of the Ancients. Hakkar's death at the hand of Malfurion Stormrage was one result of this, signifying that the old timeline was being overwritten, not relived.

AZGALOR

Azgalor was once the lieutenant to the mighty demon-king Mannoroth. Following Mannoroth's death at the hands of the orc Grom Hellscream, Azgalor took it upon himself to direct the Burning Legion's ground forces at the Battle of Mount Hyjal. Though he isn't as clever as his former master, he has an aptitude for chaos and violence that is legendary even among demonkind. Following his master's death, Azgalor took control of the Burning Legion's ground forces and accompanied Archimonde as he made his ascension up Mount Hyjal towards Nordrassil, the World Tree. After various attacks, Azgalor invaded but was repelled; after some time he began to attack alongside Rage Winterchill but was still held back.
Though the Legion was defeated that day and Azgalor seemingly vanquished, he escaped into the Twisting Nether to plot his revenge against the mortal armies of the world, seeking to destroy all life on Azeroth by employing whatever forces he could utilize.

Five years after Azgalor was defeated, dark agents attempted to alter the course of history as they tried to sabotage the Battle of Mount Hyjal. The bronze dragonflight sent adventurers to prevent it via the Caverns of Time. Because of this intervention during this battle, Azgalor didn't fight until reaching the orc base. There he fought Thrall, troops of the Third War, and adventurers from the future, but this time he was killed in the battle. It is unknown what happened to Azgalor as he survived in the original timeline but was killed in the altered timeline. It is possible that he is dead, such as Hakkar the Houndmaster survived the War of the Ancients in the original timeline but was killed in the altered timeline; another possibility is he was just defeated and later escaped to the Twisting Nether as in the original timeline.

VARIMATHRAS

A dreadlord once in the service of the Burning Legion, Varimathras was forced into allying with the Forsaken, under the direct control of Sylvanas Windrunner, in exchange for sparing his life after he and his brothers were overcome by her undead forces. He once resided within the Royal Quarter of the Undercity, standing at Sylvanas's side. Though he had sworn allegiance to her, he served a more powerful master.

During the Third War, the dreadlords (or nathrezim) were tasked with leading the armies of the Burning Legion into battle. They served as field commanders and tacticians and commanded the troops from behind the front lines. The dreadlords were responsible for the summoning of Archimonde the Defiler, who served as general for the demonic forces. It was the dreadlords Mal'ganis and Tichondrius who first spread the taint and corruption of the Legion across Lordaeron and Felwood respectively, and after their deaths by the hands of the enemies of the Scourge, it was Varimathras and his two brothers, Detheroc and Balnazzar, who were put in charge of the undead forces in Lordaeron while Archimonde took his forces across the sea to Kalimdor to siphon energy from the World Tree, Nordrassil, on Mount Hyjal.

TICHONDRIUS

Tichondrius was the leader of the Nathrezim of the Burning Legion, under the command of Kil'jaeden. He was a major character in the Warcraft III Path of the Damned, The Invasion of Kalimdor, and Eternity's End campaigns. While he appeared often, he was never available for the user to control.
Tichondrius did not appear to have any part in the War of the Ancients, though at times in WarCraft III, his comments implied firsthand experience. Since he is not mentioned in the War of the Ancients trilogy, he may not have played a significant or noteworthy role.
Prior to the Third War, Tichondrius was chosen by Kil'jaeden to personally police the newest agent of the Legion, Ner'zhul, the malefic Lich King. Tichondrius was none too pleased with the appointment, but accepted it in the end. As befitting his status as the greatest of the Nathrezim, Tichondrius was placed in charge of a number of Scourge operations. Following the corruption of Prince Arthas Menethil, Tichondrius introduced himself to the Death Knight, providing him with orders from Ner'zhul, especially concerning the resurrection of Kel'Thuzad. Because he didn't entirely trust Arthas, Tichondrius pretended to be another one of the Lich King's followers. The ghost of Kel'Thuzad, however, informed Arthas that Tichondrius and the other dreadlords were actually the Lich King's jailers.

Tichondrius regularly discussed the Scourge's progress with his colleagues, Anetheron and Mephistroth, who remained on a Legion planet in the Nether. The Darkener expressed (justified) concern that Ner'zhul had alternative plans for his new "champion," but Mephistroth pointed out that he "wouldn't dare undermine our efforts now."

The dreadlord continued his work, eventually overseeing the death of Uther Lightbringer, the invasion and destruction of Quel'Thalas, and the siege of Dalaran. Once Archimonde was summoned, he decreed that the Lich King was no longer needed, and gave Tichondrius control of the Scourge.

MAL'GANIS

Mal'Ganis was one of the nathrezim sent by Archimonde as a jailor of the Lich King. He was chosen to head up the plot to turn Prince Arthas to darkness, and make him the Lich King's greatest champion.
As Arthas and Jaina Proudmoore were investigating rumors of plague, they were attacked again and again by the forces of the Scourge posing as the personal army of the dreadlord Mal'Ganis. Kel'Thuzad helped play the part by telling Arthas that it was Mal'Ganis himself and not the Lich King behind the plague. Each attack, each town destroyed hit at Arthas' weakness, his pride. Arthas began to take his inability to protect his people from the Scourge personally, and soon became obsessed with killing the dreadlord.

Mal'Ganis led Arthas to Stratholme, and left him with the choice of either slaughtering all of its inhabitants or watching them fall to the plague. Arthas chose to kill his own subjects rather than let them become slaves to Mal'Ganis in death, killing most of the inhabitants of the town. Mal'Ganis was there in the wreckage, and goaded Arthas once again to come to Northrend to finish their battle. As expected, Arthas took the royal fleet and sailed north. The Prince's obsession with the hated dreadlord led him further down the path of evil, first lying to his men, betraying his mercenaries, and finally forsaking his own soul to take up the runeblade Frostmourne, grievously wounding Muradin Bronzebeard in the process.

The moment Arthas took up Frostmourne he became a tool of the Lich King, completely subservient to his will. This was what Mal'Ganis had planned all along. Confidently he approached Arthas and informed him of his fate. However, the manipulative and traitorous dreadlord was himself betrayed, as the Lich King's first whispers to Arthas were that the time for his vengeance had come (this was the first step towards Ner'zhul's independence). Arthas defeated the dreadlord and believed him to be slain.

MAGTHERIDON

Magtheridon, the former Lord of Outland, is a pit lord who conquered the ruins of the orcish homeworld following its destruction. His reign was ended by the arrival of Illidan Stormrage, who took Magtheridon's throne at the Black Temple for himself. Deciding that Magtheridon would have a further part to play, Illidan had Magtheridon's body dragged to the Hellfire Citadel where he imprisoned the pit lord, leaving him at the mercy of the fel orcs. A group of adventurers entered the depths of Hellfire Citadel, released the former Lord of Outland from his bonds, and finally destroyed him, taking his head as proof of their prowess.

Magtheridon was voiced by Earl Boen, a narrator for WoW's cutscenes, in World of Warcraft: The Burning Crusade.

A brutal pit lord and servant of Mannoroth the Destructor, Magtheridon found his way to Draenor after its cataclysm. With the clans in disarray, and most killed in the disaster, Magtheridon was quick to show his power, and rallied the surviving orcs under his pennant. The orcs were corrupted and became fel orcs. He declared himself the ruler of the ruined world Outland. Over the years, Magtheridon brought legions of demons to Outland through the four dimensional gateways which Ner'zhul shattered the world with, and his forces became very expansive.
Magtheridon is the boss of his lair, his rumbling cries are heard throughout the Hellfire Citadel. Strangely, he was purple in Warcraft III but now in Burning Crusade he is green like most (if not all) pit lords.

BALNAZZAR

Balnazzar (currently under the guise of Saidan Dathrohan, Grand Crusader of the Scarlet Crusade) was one of the three nathrezim siblings left behind in Lordaeron to oversee the demonic conquest of that land. The return of Prince Arthas, following the defeat of Archimonde, sent the three brothers packing, but they quickly regrouped and concocted a new plan to destroy Arthas once and for all. Though they (and Sylvanas Windrunner, who attacked Arthas on her own terms) didn’t destroy Arthas, the undead prince left Lordaeron, leaving Balnazzar to rule the Plaguelands. Soon after, Sylvanas (who forcibly made Varimathras join her) laid siege to Balnazzar's forces and ultimately forced Varimathras to execute Balnazzar. He later came to posses the body of Saidan Dathrohan, a founder of the Scarlet Crusade.
Balnazzar and his brothers, Varimathras and Detheroc, were not aware that the Burning Legion had been defeated at Mount Hyjal until months after the fact. It was only when Arthas returned from his sabbatical to Kalimdor that he announced Archimonde's destruction. Arthas stated that their deaths would complete their folly. Balnazzar and his brothers quickly teleported away before Arthas could strike.

Also, he is the final boss of the Stratholme Living Side, located within the the Scarlet Bastion in The Crimson Throne. He begins the fight as Grand Crusader Dathrohan, but eventually sheds his human disguise for his true form. It should be noted that if killed too quickly, he will not transform into his true form, and players will not receive the achievement regardless of the kill.

SARGERAS

Sargeras is the creator and leader of the Burning Legion. Sargeras was once a vanir titan, a Champion of the Pantheon chosen to defend the worlds they had created. Sargeras has changed over the millennia. While he once served his race as guardian and protector, he is now the titans' worst enemy. Sargeras works to undo what the titans have strived for ages to achieve. He is now the master of all that is evil and demonic. Lesser beings quake and flee in his presence.
Sargeras was a mighty giant of molten bronze who led the armies of the titans, the Champion of the Pantheon's cause. Like the rest of his kind, he was altruistic and just, unable to conceive of pure evil. Sargeras was called upon to defeat and imprison the hordes of demons native to the Twisting Nether, so that their evil would not contaminate the titans' vision of order. Sargeras went about his task devoutly for countless millennia.

Sargeras is a huge being almost beyond comprehension. He is fully clad in a burning suit of molten, black armor. Wildly dancing flames surround his head and form a thick mass of hair and a sweeping beard. A pair of horns exteeries of viciously hooked barbs. Demonic influences have given Sargeras body flames.
Sargeras doesn't fight fair. He fights dirty and goes for the throat as quickly as possible. He also doesn't fight alone. Sargeras has a universe of formerly vanquished demons that he now commands to do his bidding. Most of them even do so gladly.

NER'ZHUL

The Elder Shaman Ner'zhul was the Warchief of Draenor, the clans that remained uncaptured after the Dark Portal was destroyed in Azeroth. He was tricked into making a Blood Pact with Kil'jaeden the Deceiver; one that bound the orcs into the service of the Burning Legion. Following the Second War, he opened several portals on Draenor in an attempt to seek out new lands to escape to and conquer, but was immediately captured by Kil'jaeden. His mortal form was destroyed and his spirit was transformed into the spectral Lich King, which was then encased in the mystical ice of the Frozen Throne atop the Icecrown glacier in distant Northrend.

Ner'zhul was the chieftain and elder shaman of the Shadowmoon clan and one of the most popular figures in orcish society. He was admired, respected and venerated by all for his deep connection to the spirits, and was the closest thing the orcish race had to a single leader prior to the foundation of the Horde. However, deep within, Ner'zhul craved a power he did not have...
Ner'zhul had the gift of true visioning and all he had seen would indeed come to pass

MANNOROTH

Mannoroth was a pit lord whose ruthlessness and vile cunning made him into one of the favoured lieutenants of his masters, Archimonde and Kil'jaeden, the champions of the Burning Legion. Known as “the Destructor” and "the Flayer", Mannoroth was the barbaric leader of the pit lords when the dark titan Sargeras subjugated them and bent them to his will. When Archimonde was placed in command of the pit lords, he recognized Mannoroth’s value as a brutal, bloodthirsty fighter and made the pit lord his second-in-command. Mannoroth helped lead the Burning Legion in assaults on countless worlds, and led the first invasion of Azeroth.

Mannoroth was among the demons who ventured into Kalimdor during the War of the Ancients.

He singlehandedly slaughtered entire regiments of Kaldorei defenders and sowed chaos by unleashing squads of doomguards across the land. Though he battled with brutal ferocity and slaughtered many, he was defeated by Cenarius and his followers and sent back into the Twisting Nether when the Well of Eternity was destroyed.

Although his forces were ultimately defeated and forced to flee from Azeroth, Mannoroth was pleased with the level of carnage that he had inflicted.

Mannoroth was a simple being with no overriding ambition to change his station in life. Craving blood and carnage over everything else, he willingly acted as an enforcer for Archimonde and the Burning Legion. His only desire was to wage unending war against the mortal races of the universe. He was a terribly efficient weapon of destruction for the demonlords and could always be found at the forefront of their invasion forces. Mannoroth had forever been cursed with an immeasurable, unending thirst for blood. Even the bloodlust of the orcs was a minor compulsion compared to the absolute addiction that bloodletting was for Mannoroth. In the absence of mortals to fight, Mannoroth often turned on his own demon kin, slaying felhounds and infernals with an unnerving sense of satisfaction. He preferred the kill to the chase, and would quickly slay mortals who were unfortunate enough to stumble upon him. Mannoroth sought out the most concentrated area of foes and wades into battle. He was not happy unless he was spilling the entrails of his enemies at his feet. When a victim fell, he usually spent an extra attack or two on the victim making sure the target was quite dead. This could be quite messy, which was exactly how Mannoroth liked it. In fact, he could get a bit reckless if foes were not dropping to his massive two-bladed sword at least every attack.

KIL'JAEDEN

Kil'jaeden is a powerful eredar demon lord and is the acting leader of the Burning Legion. Selected twenty-five thousand years ago as the chief lieutenant of Sargeras, he was ranked above the now-deceased demon lord, Archimonde. While Archimonde had led the Legion's armies, Kil'jaeden's agenda was to assimilate every race possible into the folds of the Burning Legion. Since the apparent banishment of Sargeras, he has assumed the role of supreme commander.

He is also the creator of the Lich King.

Kil'jaeden seems to have no problem keeping his ego from dominating judgment. He is surprisingly forgiving to failed servitors, provided they aren't complete failures. Kil'jaeden possesses an extremely long-term mindset, and views individual events as nothing more than single links in the thousand-year long chains of his schemes. Perhaps because of this outlook, Kil'jaeden is extremely tenacious and rarely lets short-term defeats or missteps prevent him from reaching his goals. If one method of completing a scheme is prevented for whatever reason, he simply finds other ways to proceed. However, he also has a long memory when it comes to individuals who, either intentionally or unintentionally, obstruct his plans. Kil'jaeden does not forget those who hinder him in any way, and often comes back to brutally punish them later, in his own time. His bidding is typically carried out by minions or intermediaries, and Kil’jaeden rarely meets individuals face to face. When he does, it is usually to force them to his will or to slay them once and for all.

Kil'jaeden the Deceiver has a highly adaptable physical presence and frequently shifts among several forms. He usually tries to assume whatever shape the people around him will trust the most or be the most awestruck by. He attains his goals through a combination of subtle coercion and blatant intimidation. The exact powers at the Deceiver's command are unknown. As his ultimatum to Illidan and his appearance to Ner'zhul and the orcs exemplifies, he is a master of illusions. However, Kil'jaeden's full powers lie not within the boundaries of physical and magical ability, but in the mind. His unrivaled cunning placed him first in position in the ranks of the Burning Legion, and so aptly gave him the title of the Deceiver. He managed the corruption of the orcs with very little magical expenditure, a plan executed almost entirely by manipulation.

ARCHIMONDE

Archimonde was the left hand of the fallen titan Sargeras. Archimonde was one of the greatest and darkest of the eredar. His ruthless and cunning personality quickly earned him station and power within the eredar ranks, until finally he was one of the first among them. He is described as one of the lieutenants and generals of Sargeras.

Archimonde was extremely brutal, and often killed his underlings when they failed him, or just to alleviate his own frustration. While he seemed to be more forgiving of failures of the underlings that he couldn't replace, his brutal nature was enough of a motivation not to fail him. Although he was highly intelligent, his arrogance and his inability to keep his anger in check proved a fatal flaw. His inability to keep his ego out of the equation was exploited by Malfurion Stormrage at the Battle of Mount Hyjal.
Archimonde is the final boss in the Battle for Mount Hyjal. The fight is a major battle for survival; even a single player's death can easily result in a wipe.