The 2005 Marvel Comics crossover event House of M by Brian Michael Bendis and Olivier Coipel brought about grave consequences for the entire mutant population of the Marvel Universe. At the behest of her brother Quicksilver, Scarlet Witch had rewritten reality to one ruled by the mutants before being exposed by the New Avengers once they regained their memories. Restoring reality resulted in Marvel Merry's Mutants becoming an endangered species, but one superhero's actions would lead to other devastating consequences for X-Men.
The crossover event introduced Layla Miller, a young mutant living in Hell's Kitchen and one of the few characters that remembered reality prior to Scarlet Witch's actions. Layla revealed she had the ability to unlock other characters' memories, serving as the biggest pivotal figure in reawakening the Avengers and restoring reality. Following the event's conclusion, Layla was revealed to still be active within the Marvel Universe, and she teamed up with X-Factor as the team of mutant investigators as they reacted to the immediate fallout from House of M and the mass decimation of Marvel's mutant population.
During the X-Men crossover event Messiah Complex, Layla accompanied one of Multiple Man's duplicates to the alternate future timeline where the time-traveling mutant Bishop had originated from. While imprisoned within a mutant internment camp, the two learned more about what had made Bishop's future possible, and Layla sent that information back to the Marvel Universe's relative present while stranding herself in the alternate future. Fighting against mutant oppression, Layla grew into adulthood and went on to escape the camp and become instrumental in forming a revolutionary movement while displaying her mutant power to raise the dead, albeit without their souls -- this distinction evident by their complete lack of empathy and sociopathic tendencies.
In Peter David and Valentine de Landro's X-Factor #50, one of the first major characters resurrected by Layla with this power was Trevor Fitzroy. As the illegitimate son of Hellfire Club leader and longtime X-Men villain Sebastian Shaw, Trevor was a fellow revolutionary who rescued Layla and Multiple Man from a Sentinel attack in this future timeline, although he was visibly younger than he had been in his comic book appearances. After a rogue Multiple Man duplicate known as Cortex murdered Trevor, Cyclops' daughter in the timeline Ruby Summers implored Layla to resurrect Trevor. Despite knowing the consequences that resurrection would have decades ago, Layla agreed, setting up the soulless Fitzroy to become one of the deadliest X-villains of the '90s.
Using his mutant to open temporal portals to travel through space and time, Trevor would eventually arrive in the Marvel Universe years before the events of House of M as an older, more sinister character. After debuting in Jim Lee, Whilce Portacio and John Byrne's Uncanny X-Men #281, Trevor reprogrammed and unleashed Sentinels on the mutants, murdering all of the young mutants of the Hellions and his former love interest -- and Bishop's sister -- Shard. Trevor's attacks would render Emma Frost comatose for a time and nearly killed Jean Grey. Fitzroy was also the villain who originally caused Bishop to travel back in time, and he was ultimately killed by the X-Man in 2000's Bishop: The Last X-Man #14.
Even though Layla Miller was the most noteworthy hero to debut in House of M, she still knowingly unleashed a killer on the Marvel Universe. Whether she was guided by complex moral calculus or the history that she already knew to be true, Layla Miller still doomed dozens of mutants before she was even created.