Short answer:
In the original epic Beowulf , after the fight Grendel flees mortally wounded to his swamp (or mere) and dies there; later, Beowulf finds his corpse in the underwater lair and cuts off his head as a final trophy.

Quick Scoop: What happened to Grendel after the fight?

In the famous hall-fight scene, Beowulf tears Grendel’s arm and shoulder completely out of their socket, a wound described as fatal in the poem. Grendel, howling and bleeding, breaks away and runs out of Heorot, leaving a trail of blood behind him.

Instead of dying on the spot, he staggers back to his home in the marshes, the dark “mere” or swamp where he and his mother live, and there he dies of his injuries. The Danes hang his torn arm in the mead-hall as proof of victory and celebrate, assuming the menace is over.

Later in the story: Grendel’s corpse

Grendel’s story has a kind of grim “epilogue.”
When Grendel’s mother attacks Heorot to avenge her son, Beowulf follows her into the underwater lair. While fighting her, he eventually finds a giant sword in the cave, kills her, and then sees Grendel’s dead body lying there.

Beowulf then cuts off Grendel’s head and brings it back to Heorot as a second, even more dramatic trophy. So in terms of the narrative, Grendel’s fate is sealed twice: he first dies alone in the mere, and then is symbolically “defeated again” when his head is taken and displayed.

A quick note on other versions

Modern retellings like John Gardner’s novel Grendel tell the story from Grendel’s point of view and linger more on his thoughts and perceptions as he dies, but the basic outcome is the same: he is mortally wounded by Beowulf and dies after fleeing. Some pop‑culture or game adaptations (like certain anime/game franchises) may reinvent Grendel with different afterlives or forms, but those diverge from the original Anglo‑Saxon epic.

TL;DR:
Grendel is fatally wounded when Beowulf rips off his arm, escapes to his swamp lair and dies there; later Beowulf finds the corpse, cuts off Grendel’s head, and brings it back as a trophy.

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