Who is Grendal in Beowulf?

Grendel is a vicious, scary monster and is one of Beowulf's main antagonists.
On a nightly basis, the Danes gather at their mead-hall, Heorot, for a spot of feasting, singing, and all-round merriment. Driven almost insane by the sounds emanating from the mead-hall, Grendel takes revenge on the Danes by regularly descending on Heorot to attack and kill King Hrothgar's warriors. And he doesn't just kill them, either; he eats them too.
This goes on every night for twelve long years, during which time, as one can imagine, the Danes get pretty desperate. Brave warriors they may be, but they're simply not strong enough to take on and defeat this great hulking brute. What they need is a hero to come save them from the monster's evil clutches.
Enter Beowulf. The great Geatish warrior is the ideal candidate to slay the mighty Grendel and restore peace and stability to this troubled realm. When he arrives at the mead-hall, he immediately boasts about his ability to vanquish the cave-dwelling behemoth. In anyone else, this might be bravado or misplaced confidence. But in the case of Beowulf, it's an accurate summation of what he's capable of. In due course, Beowulf will slay Grendel—and for good measure, his mother too—showing us that his vow to kill the mighty monster was no idle boast.