The story of Grendel comes from eighth century English epic called Beowulf. The story goes as follows:
Grendel is a giant, half-man and half-monster. Every night Grendel would terrorize the hall of the Danish King Hrothgar, killing thiry of his men nightly. This started a terrible feud between Hrothgar's court and Grendel that would last many years. Each night the creature would return and murder and destroy and no one could stop him, as he was under an enchantment that made Grendel invulnerable to a sword. Eventually, Beowulf came to Hrothgar's court and swore that he would kill the beast. When Grendel arrived, Beowulf wrestled with the creature, eventually using great strength to rip Grendel's arm off. The arm was nailed up as a trophy in the court. Grendel retreated to his cave and died of the wound and of his great misery at the fact that he had lost his only pleasure: tormenting and killing those in Hrothgar's hall. After witnessing the death of her son, Grendel's mother, a beast scarcely less terrible than her son, a water-troll, determined to avenge her son. She came at night to the hall and tore down her son's arm. Beowulf followed her and saw her disappear into the waters of Dark Lake. He was attacked by serpents and brought to her cave. A terrible battle ensued and even though he had been terribly wounded by her claws, Beowulf killed Grendel's mother. Upon doing this, he noticed an enormus sword. This was the only sword that could harm Grendel. So he used it and lopped off the head of the creature and carried it back to the hall as a trophy.