Beowulf finds the lake that Grendel's Mother lives in. (Paraphrase before Section 12.)
Beowulf dives down the lake (570) and finds the lake floor (573).
Grendel's Mother finds Beowulf (574-584), attracting other sea monsters to the action (585-588).
Grendel's mother and Beowulf fight in her lair (601-619). Beowulf's Sword, Hrunting, was ineffective against Grendel's Mother (595-601). When Beowulf gets pinned down, Grendel's mother uses her dagger (620-623).
Using the special sword from the wall, Beowulf defeats Grendel's mother (633-645).
Beowulf finds Grendel's body in his mother's lair (660-661).