The pizza monster is a pizza infused with demonic possession, created by Trigon.
When Raven visited Trigon, he gave her a pizza infused with demonic possession as leftovers for when she returned to Titans Tower. The pizza grew a conscious and began consuming Cyborg's sandwiches while in the fridge. As the Teen Titans began investigating the disappearances, Robin opened the pizza box, unleashing the pizza monster. It quickly ensnared the Titans in its tentacles. Robin deduced that because the monster left Starfire's angel food cake untouched, it could potentially harm it and flung it into the pizza monster's mouth, effectively destroying it.