The beginning of this epic poem gives a very clear description of the fate of those who were in Hrothgar's hall when Grendel attacked it. The very first time that Grendel, who is described variously as a "grim spirit" and the "enemy of mankind," attacked the hall, he carried off with him various soldiers to feast upon in his lair, as the following quote makes clear:
Then he found therein a band of nobles asleep after the feast: they felt no sorrow, no misery of men. The creature of evil, grim and fierce, was quickly ready, savage and cruel, and seized from their rest thirty thanes.
After Grendel has repeated this visit a number of times, the narrator reports that Hrothgar's hall stood empty for twelve winters, because all who entered there met a similar fate. It is therefore assumed that few soldiers were brave enough to enter his hall because of the grim and grisly way in which Grendel slaughtered all who entered there. Until Beowulf arrives, few have enough courage to risk their life and nobody has enough strength to vanquish Grendel.
Those who ventured into the mead hall met the same fate as the previous visitors. They were all attacked by Grendel and none was able to defeat the creature. Grendel attacked and feasted on the warriors and all those who visited the hall at night. This went on for twelve years and later Grendel moved into the hall, stayed every night tormenting the Danes. The Danes tried to fight off the creature but it was too powerful and they were defeated at every attempt. Finally Beowulf a great warrior from the Geats visited the Danes, fought with Grendel and fatally wounded him.
That was bad enough, but the following night Grendel killed more-- blinded by sin, he felt no remorse. (You can bet the survivors started sleeping elsewhere.)So Grendel ruled, fighting right, one against many, and the greatest hall in all the earth stood empty at night. Twelve years this went on, Hrothgar suffering the greatest of sorrows.