Final answer:
Grendel stole the cup from the dragon in the epic poem Beowulf.
Step-by-step explanation:
The cup was stolen by Grendel.
In the epic poem Beowulf, Grendel is a monstrous creature who terrorizes Hrothgar's kingdom. One night, Grendel enters the mead-hall and kills many warriors while they sleep. During this attack, Grendel also steals a cup from the mead-hall.
This event serves as a catalyst for the hero Beowulf to come and defeat Grendel, showcasing his strength and bravery.