In beowulf what does Grendel do to the Thanes in Denmark

Grendel terrorizes the Thanes every night as the soldiers of Hrothgar are asleep.  He kills them and thus prevents them from staging a fight back.  The Thanes are so terrorized at the hands of Grendel for many years until a Geatish warrior named Beowulf comes to their rescue.

Explanation:

b) Beowulf was an epic poem, which chronicled the events that took place in Denmark in the early part of the 6th century CE.  Controversially, it is believed to have been authored between 700 and 750 by an unknown poet.  The main lesson from this poem is that it is rewarding to be courageous, charitable, and loyal.  As a well-deserved reward, Beowulf later became the king of the Thanes after the death of Hrothgar.