As a child, Sarah was one of the underground-dwelling Morlocks. When Callisto, the Morlocks's leader, kidnapped the X-Man Angel to be her husband, Sarah came across the captive Angel. Sarah believed Angel was beautiful. He would become an idol of worship for her.
When the Marauders decimated the Morlock community, Sarah was also personally saved by Gambit, who was not yet known by the X-Men. Years later, most of the surviving Morlocks (including Sarah) were apparently killed by Mikhail Rasputin, although they were actually brought to another dimension. There, Mikhail had established a citadel for himself on the top of a massive hill. The Social Darwinist philosophy of "survival of the fittest" was the only thing which governed the society. If someone could reach the summit of the hill they were considered "fit" and worthy of being part of Gene Nation. The former Morlock leader Callisto cared for Marrow during this period and displayed a maternal attitude towards the young mutant, although the two are not known to be biologically related. When her powers fully manifested she was forced to "up the hill", becoming more savage and killing her opponents to survive. When she managed to reach the top, Mikhail considered her fit to join his elite fighting team Gene Nation, which eventually became a terrorist group whose mission was to punish humans for their hatred towards mutants.
Marrow was later recruited by the most recent incarnation of Weapon X, who again normalized her appearance and her powers. She betrayed the Weapon X organization upon discovering their anti-mutant nature. She reformed the Gene Nation terrorist cell and, as their leader, led several terrorist attacks on Weapon X until the rest of the group was slaughtered by Agent Zero. Agent Zero left her alone, not wanting her to become a martyr, but he warned her that he would come after her again after he had hunted down the remaining members of Gene Nation.
Marrow has since come to associate with the terrorist X-Cell, confronting Siryn and M before they could assault Blob and Fatale. She still sported several bone growths on her forehead, but depowered, she's forced to use throwing knives as weapons.