Final answer:
The name of the monster who lurked under Herot Hall is Grendel.
Step-by-step explanation:
The name of the monster who lurked under Herot Hall is Grendel. In the epic poem Beowulf, Grendel is described as a fearsome and monstrous creature who terrorizes the Danes and attacks the mead hall of Herot. He is ultimately defeated by the hero Beowulf.