What greater evil does Beowulf discover?
In Beowulf, Beowulf encounters and defeats three evil forces; the monster Grendel, Grendel's mother, and a dragon.
Grendel is the first evil. Beowulf essentially acts as a mercenary in the hire of King Hrothgar, tasked with killing Grendel. Grendel has been killing and eating Hrothgar's men while they sleep. Beowulf, having a sense of honorable conduct, chooses not to use a weapon, because this would give him an unfair advantage. Instead he beats Grendel senseless and then rips his arm off. Grendel runs away and dies.
Grendel's mother is the second evil (she is not given a name). The night after Grendel is killed, Hrothgar and his men assume that peace has been restored and celebrate. Grendel's mother attacks this celebration and resumes the killings. She is tracked to a lake, into which Beowulf dives and fights her, beheading her after discovering a magical sword within her lair. Grendel's mother is not described as being necessarily more evil or powerful than Grendel, but it is significantly more difficult to find and kill her.
The third evil, the dragon, appears as a sort of epilogue, 50 years after the previous battles. Beowulf manages to slay the rampaging dragon but is mortally wounded in the process.