One of Gwen Stacy - and in an alternate timeline where Gwen returned to life, her and Peter's love caused reality to cave in on itself. Spinning out of one of Marvel's biggest crossover events ever, this particular branch reality took the saying "be careful what you wish for" to an insane new level.
In the Scarlet Witch's House of M reality, Marvel's heroes suddenly wake up to a world in which their most intimate wishes are made true and the memories of their former lives are wiped away. In Peter Parker's case, he's happily married to Gwen Stacy and his Uncle Ben was never killed. However, the Parker luck somehow persists. Racked with the nagging feeling that this is not his true reality, Peter snaps and becomes the Green Goblin as a twisted form of self-sabotage. Gwen eventually helps Peter rehabilitate, but by the end of the House of M saga, the Scarlet Witch resets reality back to its natural state.
What if? Spider-Man: House of M features a short story titled What If…Gwen Stacy Survived The House of M by Sean Mckeever and Dave Ross that explores what would happen if Gwen Stacy from the House of M Universe was accidentally brought back into the Primary Marvel Universe. Doctor Strange quickly realizes Gwen's presence is unraveling the very fabric of existence and attempts to extinguish her from their reality, but it's too late. Strange, along with some of the Marvel Universe's most powerful characters are suddenly blipped out of space and time, leaving an opening for the Masters of Evil to organize and attempt to take down the heroes once and for all.
An epic battle ensues as atoms collide and reality glitches. All the while, Gwen watches as Peter fights the Masters of Evil alongside the Avengers. As the battle rages on, Aunt May and Mary Jane get caught in the crossfire and killed right before Peter's eyes. Finally realizing the torture her presence in this universe is causing Peter, Gwen makes the choice to sacrifice her life by jumping off a nearby cliff. However, Peter notices Gwen right in that moment and jumps after her, effectively saving Gwen by breaking her fall with his body. While Peter slowly dies, Gwen kisses him one last time as reality fades to nothingness.
As heartbreaking as this story's end is, it displays a Romeo and Juliet kind of irony in its final tragic moments. As far as Peter could discern, he was finally absolving himself from one of the greatest mistakes of his life by saving Gwen as she plunged to her death. Despite this personal win, Peter was completely unaware of the fact that this heroic action would be the cause for the collapsing of all reality. Still, as sad as this conclusion ultimately is, there's something undeniably beautiful about Spider-Man and Gwen Stacy reuniting for one final brief moment of love before all existence was snuffed out forever.
