"You! Ridiculous hooded one!"
Yog-Sothoth is an immensely powerful, demonic being from another dimension who appeared in Big Trouble In Billy's Basement.
His work is immediately recognized by Hoss Delgado, who reveals Yog is from another dimension and that "bad stuff is about to go down."
Billy then comes down and proceeds to fight the group, until Grim steals The Bad Book back to figure out how to get rid of Yog, by throwing the book into the portal.
However, Billy swallows the book, and is seemingly triumphant, until Mandy pushes Billy into the portal to get the book into it. A few seconds later, Yog spits Billy out.