The Soul-Sucking Death Worm is a colossal monstrous worm that appeared in the episode Dead at Eight. After Mordecai and Rigby reads The Hungry, Hungry Soul-Sucking Death Worm to Thomas, they pull a tab which releases the Soul Worm. It appears that it eats mainly, or just, souls.
It is a giant reddish-orange worm with a giant mouth with little waving pieces of skin and no eyes. The bottom of the worm is just a lighter shade of red-orange. It's actually normal for worms to be eyeless.