Throughout Doctor Strange in the Multiverse of Madness, the dangers of tapping into dark magic are made clear to Stephen Strange. As well as witnessing the Scarlet Witch's downfall, he learns of two Variants who caused the destruction of entire universes after incursions (which take place when two different realities smash into each other and only one of them survives).
Trapped in a doomed reality, the former Sorcerer Supreme is left with no other choice than to use the Darkhold in order to "dream walk" into the MCU and take control of Defender Strange's corpse.
Ultimately, Strange succeeds in saving America Chavez and she makes Wanda Maximoff see the error of her ways by exposing her Variant's children, Tommy and Billy, to this deranged version of their mother. Realising what she's become, the Scarlet Witch destroys every copy of the Darkhold throughout the Multiverse and buries herself beneath the tomb where it was created in the MCU. Does she survive? We never find out, and the fallen hero's fate is left deliberately ambiguous.
America ends up training to be a sorcerer at Kamar-Taj, while Strange returns to the Sanctum Sanctorum. The hero toying with dark magic doesn't seem to have too much of an impact on him until he's walking down the street and a third eye, similar to Sinister Strange's, opens on his forehead.
This could be the beginning of the end for Strange, especially as a mid-credits scene shows him leaping into the Dark Dimension with Clea after she asks for his help in fixing an incursion. At this point, that third eye opens on his head, suggesting Strange has started embracing his darker impulses.
It's clear the premise of Doctor Strange 3 will be a team-up with Clea, and it wouldn't be surprising for Dormammu to make his grand return. As for Stephen, we can't help but wonder if embracing dark magic is going to send the Avenger down a bleak path he'll be unable to return from. As for those incursions, that could be Marvel Studios' way of setting the stage for a Secret Wars movie, with multiple realities smashing into each other and leaving us with a brand new MCU...