Once upon a time, there was a scary monster named Grendel who lived in a far-away land. Grendel caused a lot of trouble for people because he liked to attack and eat them. One day, a brave warrior named Beowulf came to fight with Grendel and he defeated him.
But then something very sad happened, Grendel's mother, who was also a monster, got very angry and came to take revenge for her son's death. She was very scary and strong, and she also wanted to eat people.
Beowulf knew he had to fight again, but this time it was going to be even harder. Grendel's mother lived in an underwater cave, which meant that Beowulf had to dive into the water to face her.
When Beowulf found Grendel's mother, she attacked him, and they had a fierce fight. But in the end, Beowulf was able to defeat her too, even though it was very hard.
So, in summary, Grendel's mother was a monster who was very angry when her son was defeated by Beowulf, and she wanted revenge. She was also very strong and lived in an underwater cave, but Beowulf was able to defeat her in the end.