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.