The relationship between Grendel and Hrothgar is one of hate; Grendel loathes the celebrations of God that take place at Heorot. Though Beowulf and Hrothgar had no personal relationship before Beowulf arrived to slay Grendel, Hrothgar knew Beowulf's father, respected him and believes that Beowulf might be able to help save Heorot.
The narrator in Beowulf describes Grendel as a monster who was cast out from the sight of men. As Hrothgar builds Heorot and the men gather to enjoy their feasts and tales, Grendel stirs. The narrator describes Grendel, saying:
Grendel this monster grim was called,
march-riever mighty, in moorland living,
in fen and fastness; fief of the giants
the hapless wight a while had kept
since the Creator his exile doomed.
On kin of Cain was the killing avenged
by sovran God for slaughtered Abel.
Ill fared his feud, and far was he driven,
for the slaughter’s sake, from sight of men.
Of Cain awoke all that woful breed,
Etins and elves and evil-spirits,
as well as the giants that warred with God
weary while: but their wage was paid them!
Grendel arrives at Heorot and kills many of the men before returning to his lair. The men continue to use Heorot as a mead hall to praise God and tell stories of their own glory; Grendel returns to kill them. Grendel's actions bring Hrothgar great sorrow. The narrator says:
There came unhidden
tidings true to the tribes of men,
in sorrowful songs, how ceaselessly Grendel
harassed Hrothgar, what hate he bore him,
what murder and massacre, many a year,
feud unfading, — refused consent
to deal with any of Daneland’s earls,
make pact of peace, or compound for gold:
still less did the wise men ween to get
great fee for the feud from his fiendish hands.
This continues until the mead hall is abandoned since no one seems able to kill Grendel.
When Beowulf arrives to try his might against Grendel, he tells Hrothgar of his past exploits and successes fighting monsters. Ultimately Hrothgar is so sure of Beowulf's goodness and strength that he yields control of Heorot to Beowulf for a night while the men wait to fight Grendel, something he's never done for another man before. Hrothgar says:
Never to any man erst I trusted,
since I could heave up hand and shield,
this noble Dane-Hall, till now to thee.
Have now and hold this house unpeered;
remember thy glory; thy might declare;
watch for the foe! No wish shall fail thee
if thou bidest the battle with bold-won life.
Though Grendel and Hrothgar are enemies and locked in a feud that brought the king years of grief, the arrival of Beowulf gives Hrothgar hope for the first time and, ultimately, allows him to regain control of Heorot.