Which second enemy does Beowulf fight in an underwater lair?

Beowulf fights Grendel’s mother in an underwater lair. After Beowulf kills Grendel, Grendel’s mother attacks Heorot to avenge her son and carries away the severed arm that had been displayed in the hall.

Beowulf follows her tracks to a mysterious mere. He dives into its depths, where hostile water creatures attack him, and discovers a cavern beneath the water. There he battles Grendel’s mother with a sword found in the chamber and finally kills her.

A common mix-up is to treat Grendel’s mother as simply another version of Grendel. The poem presents her as a separate enemy motivated by revenge, and her fight forms the second major monster episode. The underwater setting also distinguishes this battle from Beowulf’s first confrontation, which takes place inside Heorot.