While Incursions leading up to Marvel's Secret Wars, as the powerful heroes of the Illuminati took it upon themselves to destroy alternate reality versions of Earth in order to save their own.
When the Beyonders grew tired of reality, they set off a cascading disaster in which alternate versions of Earth were pulled into each other's orbit, smashing together to destroy both realities. When Reed Richards, Black Panther, and the rest of the Illuminati learned of this, they devised various ways to stop the Incursions, including destroying the parallel Earths that spelled certain doom for the planet. While the first few Incursions saw dead planets appear in the sky, the heroes knew that sooner or later, they'd have to face another Earth's protectors.
That moment came on Earth-4290001, a thinly-veiled version of the DC Universe. This world was protected by the Great Society, including the solar-powered Sun God, the human vigilante Rider, super-fast Boundless, shape-shifting alien the Jovian, helmet-wearing mystic the Norn, and light-manipulating Doctor Spectrum - equivalents to Superman, Batman, Flash, the Martian Manhunter, Doctor Fate, and Green Lantern respectively. After Namor pushes the teams into a direct confrontation, the Great Society immediately prove to be more powerful than the Illuminati, with Sun God in particular downing the Hulk and turning the tide of battle. That is, until Doctor Strange unleashes a spell that horrifically kills each member of the team, and leaves Sun God a dying husk. The battle takes place in New Avengers #19, from Jonathan Hickman and Valerio Schiti.
The series reveals that while Doctor Strange attempted to bargain for power by trading his soul, his activities with the Illuminati have twisted his morals so badly, he no longer has enough of one to be worth taking. Instead, Strange turns to forbidden tomes, learning to summon an eldritch creature that destroys a team of heroes just trying to protect their world. It's a complete abandonment of every heroic instinct Doctor Strange has ever possessed, and begins a moral spiral that ends with him becoming Doctor Doom's second in command in Secret Wars itself. Sadly, this original version of Strange is killed by Doom during the event, but that at least means that when a new version of reality is created, its version of Stephen Strange is innocent of this horrifying crime.
Doctor Strange deals with threats far in excess of what other heroes have to reckon with, often facing horrors that leave no room for mercy. As he says following the death of Marvel's alt-reality Superman, he has always been a student of the "dark arts." Pushed to his extreme by the end of the multiverse, Strange's morals buckled and broke - a fall from grace Marvel represented by the Sorcerer Supreme slaying its competitor's greatest heroes, including an obvious facsimile of the ultimate symbol of hope. The lead up to Secret Wars focused on how an unwinnable fight broke a group of men who considered themselves great heroes, and Doctor Strange feeding Earth-4290001's Justice League and Superman to an eldritch abomination was his personal nadir.
