Astagoth the Devourer is a creature mentioned in the Dan Vs. episode "The Monster Under the Bed".
It is mostly based on Lovecraftian lore, and resembles Cthulhu.
Dan swore vengeance on what he believed was a monster under his bed after it locked him out of his house. Checking under his bed, Elise discovered a scale that could prove that the "monster" wasn't just Dan's imagination. As it turns out, the creature was Astagoth the Devourer, who Dan accidentally summoned after putting a dent in Chris' car. In order to send Astagoth back to its own dimension, Dan was forced to repair Chris' car, and then travel to a spot in the Canadian wasteland (still covered in snow from the episode "Canada"), strip down to his underwear, cover himself in a special ointment and grass, and read an incantation. Then it was revealed that the whole thing was actually an elaborate plot by Chris and Elise to get Dan to leave them alone for a few days. The episode ended with Dan swearing vengeance on Chris.