In the epic Beowulf, what large event happens fifty years into Beowulf's reign?
Concerning Beowulf, I believe your question refers to Beowulf's battle with the dragon and his resulting death.
Beowulf performs his last great feat by killing the dragon that has been plaguing his people. In the process of doing so he himself is killed. Of course, being an epic poem, the hero, Beowulf, has time to give a speech before he dies.
He asks his loyal subject--the only one to stick by him during the battle with the dragon--to build a tower to serve as a beacon for sailors and a remembrance of Beowulf.
Beowulf's killing of the dragon and his resulting death are the major events that occur at the end of Beowulf's reign.