One of the most startling revelations from House of X and Powers of X has been the secret role Moira McTaggart has had throughout X-Men history. The character was retconned to have always a mutant, capable of reviving herself after death into a new timeline.
Strangely enough, her new abilities line up with the origins of one of DC's deadliest characters. Moira's revised history shares some serious similarities with Doomsday, the hulking monster that did the impossible and killed Superman. Beyond their shared surface similarities, they also pose existential threats to the worlds of their respective heroes.
Both Moira and Doomsday derive their power from the time of their deaths - plural. The original origins of Doomsday revealed he was a test subject of a scientist called Bertron. Attempting to create the perfect lifeform, Bertron genetically engineered an infant that he then sacrificed to the natural elements of an ancient Krypton. Every time the lifeform was killed, Bertron would recover the corpse and create a resistance to that within a clone of the lifeform, before repeating the process for decades.
Ultimately, this led to a creature that was resilient to almost all forms of pain or death, and consumed by a never-ending hatred for life itself. It would tear a bloody path through the universe before landing on Earth. There, it battled, and even proved capable of standing up to Superman. Beyond even it's natural defenses if Doomsday is killed in the present day his body will restore itself, adapting and becoming immune to the means that last killed him.
While Moira doesn't gain any additional physical strengths from being resurrected, her powers of resurrection also give her a major advantage. Moira is able to retain her knowledge and memories between lives. She's been killed multiple times only to come back her knowledge from the previous life, giving her a wealth of valuable knowledge. This has benefited her scientific work, and given her an awareness of what people to recruit and which to target.
While Doomsday was, from the outset, a threat that needed to be contained, Moira is a long-time ally of the X-Men. Her recently revealed powers complicate things though, especially considering the impact she's having on the team and their mission. In the same way that Doomsday was the "Death of Superman," Moira might be the final piece needed to erode and kill the ideal behind the X-Men as a concept.
X-Men has always had a fascination with villains who take their dedication to protect the future too far. Magneto and Apocalypse are both prime examples of men who define themselves to "higher ideals" that lead to terrible crimes. This allowed the X-Men to be activists for peace, preventing war on both sides of the genetic divide. That's what made them the X-Men, protectors of a world that hates and fears them, all to prove that they shouldn't. And now both of those villains have been revealed to have worked with Moira at some point in her development. She's given more than anyone else for the mutant cause, literally dying multiple times for her beliefs. Now she's convinced Charles Xavier and the X-Men to work with many of their enemies, all for a "common good" that the world is clearly not prepared for.
It's already been shown that other superhero teams like the Fantastic Four are concerned with the actions of the X-Men. This could damage the team's reputation even more than it has been in the last decade, which has seen the team outright go to war with fellow heroic groups like the Avengers and Inhumans. Is Moira pushing the team to the kind of extreme that they used to combat themselves? Her actions have already led to numerous deaths, including many X-Men. If so, then there might come a point where she asks someone to do something morally wrong for a "greater good", and doom the X-Men to become the thing they used to stand against.
Moira may be the X-Men equivalent of Doomsday in more than one way - she could be the death of the dream that birthed the X-Men, becoming the most effective and dangerous villain the team has ever dealt with.