The Geats were attacked 50 years into Beowulf's reign?

In the fiftieth year of his reign, another monster has the Geats under attack. A slave stole a cup from a fire-breathing dragon's treasure trove. This dragon was guarding the treasure, which was left by an ancient civilization. The last member of the race has a particularly moving speech in which he realizes that life is fleeting, compared to the permanent wealth. Eventually the dragon found the treasure, and he has guarded it for three hundred years. He slept in peace until the slave stole the cup as a plea for mercy from his lord. Now the dragon realizes that something is missing, and he goes on a rampage to find the cup.