Grendel was a monster, one of a giant race which survived the great flood,
slain by Beowulf. It is told that his origins stretch
back to Cain, who killed Abel. He is of particular cause of trouble to Hrothgar because of his disregard for law and custom: he refuses to negotiate a peace settlement or to accept tributes of gold.
There is reference to "Grendel's Mere", "Grendel's Pit" and "Grendel's Peck" in the Anglo-Saxon Chronicle. The references seem to collaborate the underground or water lair of the Beowulf epic, but it is unclear what the true origins of these names were.
Grendel's mother is supposedly a smaller creature than her son. She is a vengeful creature who illustrates the constant cycle of war in the poem, even when the enemy appears to be defeated.
As part of a mythical giant race, both Grendel and his mother appear impervious to normal swords, hence the difficulty the Danes must have had in trying to deal with them. Beowulf eventually finds a sword forged by the giants themselves in order to defeat them, but their blood runs hot enough to melt even that blade.