In 500 AD, in Denmark, the monster Grendel came to plague Heorot, the hall of Hrothgar, King of the Danes. Beowulf came to oppose Grendel and, when he returned to the king's hall, tore off one of his arms, apparently killing him.
Grendel's mother, enraged by the death of her son, emerged from her swamp and traveled to Heorot, dragging off and slaying Aeschere, a trusted advisor (and possibly the chief warrior) of Hrothgar. Beowulf tracked the mother back to her swamp, dove into her lair, and seemingly slew her using a god-forged sword.
It is later revealed that the mother of Grendel was not actually slain, but rather magically imprisoned in the ground below Denmark. Over the centuries, her prison migrated across the planet, until, in the modern era, it existed below Los Angeles, California.
Big Mother was freed from her imprisonment when Merlin Demonspawn spilled the blood of 100 virgins in his comic book store.
Marlo Jones managed to talk Big Mother out of eating her employee Al. She then bought more time by starting a quarrel between Big Mother and Merlin. The wizard wanted her to lead him to the Holy Grail which had been guarded by a dragon who was yet another of Beowulf's foes. However, Big Mother refused to be used by Merlin. The two were about to fight when Merlin was attacked by Al wearing the armor of the Red Raven. Al was downed saving Marlo, but Captain Marvel arrived in time to engage Merlin.
Big Mother wandered out during the battle and briefly encountered Moondragon without incident. She then encountered Rick Jones' former agent Mordecai Boggs. Boggs, apparently oblivious to her monstrous nature, talked her into taking him on as her agent. Big Mother was last seen on a promotion for FWW wrestling, vowing to eat her opponents alive.