In Doctor Strange in the Multiverse of Madness: Why did Doctor Strange become a zombie? Let’s explain.
Strange uses the Darkhold to prevent Wanda from stealing Chavez’s talents while Doctor Strange and Christine 838 are locked in the netherworld reality. He used the magical practice of dreamwalking, in which a person enters a parallel world version of themself. Because Wanda is evil in reality 616 and he’s Strange 616, he must take on the form of the deceased version of himself in that dimension to stop Wanda.
However, it appears that this is not without repercussions, as spirits assault him because magic users are not permitted to resurrect the dead. From her end, Christine aids in their management, allowing Strange to employ them as a cover in his battle against Wanda. However, it appears that employing this black magic will have consequences — it appears to produce an “incursion.”