BIOGRAPHY - Page 1
Marrow wasn't always the hateful, violent person she was when she first encountered the X-Men. She started out as an innocent child named Sarah, thrown into violence, not for being born different, but for being "scientifically useless" in the eyes of a madman. Sarah belonged to the outcast group known as the Morlocks, an underground community of mostly hideous mutants living in abandoned parts of New York's subway system, the main tunnels being dubbed "the Alley." The group, although they were not aware of it, were the subject of the Dark Beast’s genetic experiments, though it is unknown whether Sarah was one of these subjects, or actually born through subjects.
Sarah’s first interactions with the X-Men came at a time when she knew nothing of the world outside her tunnels. Callisto had taken captive one of the founding members of the X-Men, Angel, to be her husband, because she deemed him to be the most beautiful man in the world. Upon seeing the captive Angel, Sarah believed it as well, thus starting a sense of worship for the highflying X-Man. [Uncanny X-Men (1st series) #347]
Being raised in the Alley, young Sarah lived there as happily as a Morlock child could, until the day the Marauders entered the tunnels and proceeded to massacre her friends and family. At the request of his employer, Mr. Sinister, the mercenary known as Gambit had assembled this team of fellow mutant mercenaries and led them straight into the Morlock tunnels. The evil scientist had recently discovered that the Morlocks had been upgraded with genetic techniques derived from his own work and wanted this unauthorized use of his work erased. [Cable (1st series) #28]
[Note: Actually the Dark Beast had learned genetics from Mr. Sinister in his native timeline, the Age of Apocalypse. Apparently, his counterpart's studies were similar enough for Mr. Sinister to believe that the Dark Beast's experiments were based on his own.]
Gambit had been unaware of the group's true mission, though, and as he saw the horror that he had inadvertently caused, he knew he had to attempt to stop it. However, as soon as he tried to intervene, he was quickly stopped, and nearly killed, by Sabretooth. As there was no way he could win this fight, Gambit decided to run, though he managed to save one life at least. Seeing one little girl with bones sticking out of her face, Sarah, Gambit picked her up and took her along. [Uncanny X-Men (1st series) #350] As they ran through the tunnels to escape, they came upon the crucified Angel, his wings pinned by one of the Marauders' harpoons, and Sarah couldn't help but think that heaven had been brought down upon her beautiful Angel. [X-Men Unlimited (1st series) #18]
Apparently, Gambit left Sarah in the care of some other Morlocks who had survived the massacre, for she was still living in the Alley later on, after the survivors had moved back into their old home. Months later, Mikhail Rasputin, the somewhat deranged brother of the X-Man Colossus, decided to attempt what the X-Men viewed as a second genocide of the Morlocks by flooding their Tunnels. Sarah was playing in the tunnels, but was frightened by the rumblings the floodwater had created and hid in a small alcove, where she was spotted by fellow Morlock Thornn. She tried to reach Sarah and help her, but more water rushed in, sweeping the young girl away. It seemed to have been the end of the Morlocks, but secretly Mikhail had just transported most of them to another dimension.
One of the few that were left behind was Thornn. The walls between dimensions already weakened, Sarah was able to see from the other side that Thornn was searching the tunnels for any survivors, even asking Cable and Domino of X-Force to help her. Sarah managed to whisper a message that was heard by the three, saying that the Morlocks were still alive, but too "afraid to come out." Supervised by Thornn, Cable and Domino performed a Morlock ritual called the Ceremony of Light, in which the entire Alley is covered with light due to a number of cleverly arranged mirror shards. Meant to show the Morlocks that it was safe to return, it was only Sarah who crossed over the border and greeted the uneasy search party. She thanked the three for performing the ceremony, but explained that the others were still too scared to come out. As she left again to join her comrades, Sarah promised that she would return to Earth someday. [Cable (1st series) #15]
The dimension that Mikhail had transported the Morlocks too was called the Hill. It was a rather harsh realm, some sort of toxic wasteland, and it was there that young Sarah lived for the next 10-15 years. Life on the Hill was a struggle, where only the strong would survive. 
Callisto cared for Sarah as she grew on top of the Hill, until the day that her powers fully manifested and she was banished to the Outlands at the Hill’s bottom, like everyone else. Mikhail established a hierarchy based on Darwin’s survival of the fittest theory. To prove their worth, the Morlocks had to climb uphill and face whoever they met on the way. The higher one got, the less space there was and the deadlier the opponents became. Only those who were brutal enough to kill everyone in their way would reach Mikhail’s fortress on top of the Hill, where they were greeted by cool air and clear skies. When Sarah finally reached the top of the Hill, she had reached young adulthood and taken on the name Marrow because of the jagged bones that erupted from her skull, arms, back and legs. Sarah proved herself to be savage enough to remain, and was inducted into the ranks of Gene Nation, Mikhail’s personal fighting elite. [Uncanny X-Men (1st series) #374]
Eventually, Mikhail turned Gene Nation into a terrorist group, their mission being to exact revenge on humans for their hatred towards mutants and also to punish the few survivors 
of the original Morlocks for hiding underground instead of fighting back. As the team’s apparent leader, Marrow was sent to Earth first to clear a path. To her surprise, time had moved a lot faster on the Hill and, while she had been away from the Morlock Tunnels over a decade there, on Earth only a few months had passed. However, that was not the only surprise in store for Marrow. Right after performing the Ceremony of Light all by herself, she ran into the Dark Beast, whom she recognized as the “first among us,” obviously in reference of him being the Morlocks’ secret creator. McCoy took quite an interest in Marrow and her teammates, and followed their next exploits. [X-Men Prime]
Right after the other members’ arrival, Gene Nation started their acts of terror. First, they massacred all people attending a 
nightclub in New York, before targeting former Morlock, Leech, now in the care of Generation X’s headmistress, Emma Frost. Marrow and her comrade, Hemingway, delighted in torturing Emma Frost, who they considered helpless with Leech involuntarily negating her powers, but Emma proved herself to be rather resourceful and escaped. Having observed the battle from afar and rather upset that Marrow has meddled too much, the Dark Beast created a diversion for her and Hemingway to escape before they were captured by Generation X. [Uncanny X-Men (1st series) #322, Generation X (1st series) #5-6]
Gene Nation’s made their next move on the anniversary of the Mutant Massacre. Taking an entire subway train hostage, the angry mutants lured the X-Men into the tunnels beneath the city. Marrow had her teammates keep the rest of the X-Men busy, so that she could face Storm on her own. In fact, she was quite mad at the former Morlock leader, for when she was still a child, Marrow had hoped that the 'bright lady' would lead them from the tunnels. However, Storm never did that, more or less abandoning the underground community. During their duel, Marrow cruelly revealed that she had attached a detonator to her heart, which would set off the explosives the hostages had been attached to. Her heartbeats serving as the bomb’s counter, Marrow anticipated that the X-Men’s leader would hesitate too long, because of her vow never to kill. Quite on the contrary though, Storm immediately realized that there was no other choice and ripped the still beating heart from Marrow's chest, safely disarming the bombs in the process. [Uncanny X-Men (1st series) #325]