After slaying Grendel, the title character of this epic poem becomes King of the Geats & rules for 50 years
The poem relates his heroic exploits over 50 years, including the fights with .... 
Hrothgar's minstrel sings songs of Beowulf and other great characters of the past, 
... soon after, so Beowulf becomes king of the Geats and rules the kingdom well.