Complex sentence: Hrothgar tells both Beowulf and Wiglaf, who had been sleeping outside the hall during the attack, that it was the work of Grendel's mother, the last of the Water Demons, who was thought by Hrothgar to have left the land.
Split into simpler sentences: Hrothgar tells both Beowulf and Wiglaf, who had been sleeping outside the hall during the attack, that it was the work of Grendel's mother. She is the last of the Water Demons, who was thought to have left the land.