However, during the following night Grendel’s Mother arrived to avenge the death of Grendel and collect weregild. As Beowulf slept in a different building he could not stop her. He resolved to descend into the bog in order to kill her. They fought beside Grendel’s corpse, and Beowulf finally won with the aid of an enchanted giant sword stolen from the lair’s plunder. Beowulf was recompensed with gold and horses by queen Wealhþeow, and they returned to Geatland.

Having returned, Beowulf took part in a historic raid against the Franks with his king Hygelac. Hygelac died during the raid, and Beowulf swam home in full armour. Back in Geatland, queen Hygd offered Beowulf the throne but Beowulf declined in favour of the young prince Heardred. However, Heardred received two Swedish princes, Eadgils and Eanmund who reported that they had fled their uncle Onela who had usurped the Swedish throne. This led to a Swedish invasion in which Heardred was killed. Beowulf was proclaimed king and decided to avenge Heardred and to help Eadgils become king of Sweden.

The event when Onela was slain was probably a historic event. Even though it is only briefly mentioned in Beowulf, it occurs extensively in several Scandinavian sources where it is called the Battle on the Ice of Lake Vänern. For example, Snorri Sturluson wrote: Onela rode Raven, as they rode to the ice, but a second one, a grey one, hastened, wounded by spears, eastwards under Eadgils. […] In this fight Onela died and a great many of his people. Then king Eadgils took from him his helmet Battle-boar and his horse Raven (although, in Snorri’s text the names are in their corresponding Old Norse forms).

Beowulf ruled the Geats for 50 years, until his realm was terrorized by a dragon whose treasure had been stolen from his hoard in a burial mound. He attacked the dragon with his thegns, but they did not succeed. Beowulf decided to follow the dragon into its lair, at Earnanæs, but only his young Swedish relative Wiglaf dared join him. Beowulf finally slew the dragon by cutting him in half with a dagger, but was mortally wounded by being stabbed with the poisonous horn of the dragon and was carried out by Wiglaf. He died after prophesying that the Swedes would attack the Geats once again. He was buried in a barrow by the sea. The Swedish scholar Birger Nerman has proposed Skalunda Hög (58°33′N, 12°59′E) in Västergötland (West Geatland) to be Beowulf’s mound.