Answer:
monstrous.
Step-by-step explanation:
Grendel is one of the three antagonist's in the epic Beowulf. Grendel, in the epic, has created a havoc in Heorot, making all of the city's citizen fearful of him.
Grendel is mainly seen as monstrous and a giant. Grendel is described as a monster, a giant, and a descendant of Cain in the epic. His monstrous nature has been described more elaborately by the poet by stating that he consumes men after killing them.
So, the correct word to complete the sentence is monstrous.