Grendel is a great monster and one of three antagonists of the titular character of the poem Beowulf. He and his mother (one of the other three antagonists) are noted to be descendants of The Holy Bible'sCain, the first murderer.
Grendel in Halls of Lost Legends is one of four monsters summoned by Loki to fight the Ninja Turtles. His appearance here is that of a large, red, bull-like creature. He initially is taken on by Donatello, but the demon is so large and powerful that eventually his brothers are forced to assist.
Scholars refer to the anonymous Anglo-Saxon poet who wrote the epic poem as the "Beowulf poet".