Which monster kills Aeschere the king's closest friend?

Aeschere, the king's closest friend, is killed by Grendel's mother in the epic poem "Beowulf." Grendel's mother seeks revenge for the death of her son, Grendel, who was slain by Beowulf. Aeschere's death prompts Beowulf to venture into her underwater lair to confront her and seek retribution.