Grendel kills thirty men on his first attack on Heorot. The Old English poem describes the monster entering Hrothgar's mead hall at night, seizing the sleeping warriors, and carrying thirty of them away to devour them.
This attack begins Heorot's long period of terror. Grendel returns repeatedly for twelve winters, making the hall unusable and driving Hrothgar and his people into despair. The violence is especially shocking because Heorot represents communal life: it is a place for feasting, gift-giving, music, and the public display of a king's power.
The number thirty is sometimes confused with another famous number in the poem. Beowulf is praised as having the strength of thirty men in his grip, but that comparison refers to the hero's physical power, not to Grendel's first group of victims. Later, when Beowulf confronts Grendel, he fights without weapons because ordinary swords cannot harm the monster. Their struggle ends with Beowulf tearing off Grendel's arm, turning the hall's recurring nightmare into a heroic victory.