What does Grendel's mother grab beowulf and carry him to?

She carries him to her “hall,” a cave protected from the water.

They fight in her underwater lair.

"As he nears the murky lake floor, Grendel’s mother senses his approach. She lunges at him and clutches him in her grip, but his armor, as predicted, prevents her from crushing him. She drags Beowulf to her court, while a mass of sea-monsters claws and bites at him. Beowulf wields Hrunting, the sword lent to him by Unferth, and lashes at Grendel’s mother’s head, but even the celebrated blade of Hrunting is unable to pierce the monster’s skin. Beowulf tries to fight the sea-witch using only his bare hands, but she matches him blow for blow. At last, he notices a sword hanging on the wall, an enormous weapon forged for giants. Beowulf seizes the huge sword and swings it in a powerful arc. The blade slices cleanly through the Grendel’s mother’s neck, and she falls dead to the floor, gushing with blood."

They eventually fight in the lair when Beowulf tracks them down but initially when they attack the Hall (what I think the question asks) Grendel's mother carried him back to the underwater lair.