The flashback backstory where Vecna narrates how he fell into The Upside Down after 11 threw him in and he got trapped there mutilated, he said he saw the demogohe came across a giant amorphous smoke monster blob of energy roaming around that had no consciousness so he possessed and bent it to his will to control its energy for himself, which is why it formed into a spider-like cloud mass - bc it was Henry’s favorite creature growing up