Grendelís mother goes to Herot to seek revenge the night after Beowulf defeats Grendel.
Answer: The night after Beowulf defeats Grendel, Grendelís mother goes to Herot to seek revenge.
Thats my answer but somehow I feel it isnt a correct inversion or switch of the sentence. I feel the comma is used incorrectly but shoul dbe there for clarity or to stop confusion, i dont think a smeicolon is needed since these ar etwo different ideas but putting a period wouldn't work either, any suggestions?
The comma's OK, but optional. The longer the time phrase, the more likely we are to use a comma, and here it's a fairly long ohrase, so I'd use it. We could replace 'Grendel's' with 'his' (wasn't he male?).