What effect does it have on the story that Grendel only seems to attack Herot at
night? *

It makes the story to be filled with suspense.

Explanation:

From the Beowulf story, the evil monster Grendel holds Denmark hostage because it has been attacking the towns within and destroying things, and none of the weapons the warriors have is able to do any damage to it.

Grendel attacks mead-hall Herot at night because he is totally evil and he hates to hear the happy laughter of men in the hall. He attacks them at night, kills many, leaves, and comes back another night to continue his torment.

This tactic makes the story filled with suspense because nobody knows what will happen next.