Grendl’s mother kills the king’s strongest warrior before retreating. Beowulf and the other warriors track her to her lair under a lake. The lake is described in the poem as black and terrible; so forbidding that a twelve point stag will die on its banks torn down by the hounds rather than leap into those waters. In order to kill the monster Beowulf has to do just that; leap into the water and dive to the bottom of the lake where the monster waits.
All of this is of course a metaphor. The monster doesn’t come when we’re awake, when we’re in company or partying with our friends, it comes in the deep dark of night. We can fight it and kill it, but it will still return to murder the best part of us. And in order to destroy what gave birth to it, we have to dive down into the deepest and darkest waters inside ourselves.