Well in order for a Lunar eclipse the sun, earth, moon must line up in that order and the moon must be in the umbra. For a solar eclipse the sun, moon and then earth must line up. hope this helps!
I think it's clarity, the story has to be clear so that the readers understand.
Grendel is depicted as an evil mindless monster whose only goal is destruction and spreading evil. Considering that he's not depicted as an intellectual tactician but rather as a brute who wants to eat others, he would probably just heal up and attack again. Either that or he would ask for help from his mother.
