In the Epic of Beowulf, or simply Beowulf, King Hrothgar builds Heorot. Heorot is the mead hall in which Hrothgar and the multitude of tribes can come together and celebrate Hrothgar's victories (which were insured by God). In order to thank God for his successes, Hrothgar sends out word to all of the tribes of the earth that he is going to build a mead hall. Numerous tribes arrive to build the greatest mead hall on earth. At this point in time, Hrothgar is happy. He has no foes, all of mankind have come together to help him celebrate God, and his hall is renowned.
Unfortunately, a dark monster is at large. Grendel, exiled from God's light for being a kin of Cain, despises God and all those who worship him. He enters into Heorot in order to instill fear upon Hrothgar and his men. The very first night of his attack, Grendel murders thirty of Hrothgar's men. Grendel ends up killing many more men after his first attack on Heorot.
Therefore, King Hrothgar is driven to despair because of the numerous lives Grendel takes. He is also discouraged because he is unable to end Grendel's wrath.