Could just give Chthon a Mojo role, but much grimmer and scarier. Wanda is thrown into a story setting created by Chthon through Darkhold.
And Wanda is essentially a character in that story, initially bound by the rule set by Chthon, playing his twisted game. (Chthon could just casually feed other unfortunate souls to N'Garai just so Wanda would want to surrender and give him access to Earth.)
Then she would realize something from Chthon's own words or others clues that she is in fact granted a shard of Chthon's power, thus she doesn't need to be the character of the story, but a writer as well. She has the power to decide the story for herself.
Also the second spoiler tag is [/spoil].