Because it is a long narrative poem, the mood shifts throughout the work, depending on the action or purpose of each scene. It is certainly a mood of terror and suspense when Grendal attacks the mead-hall and kills yet another of Hrothgar's men, and a somber mood the next day when the men view the aftermath of what happened the night before. It is a perhaps cautious mood of relief when Beowulf shows up and announces that he has come to battle with the monster. Hrothgar and his people are wearied by the constant attacks, but chagrined that they themselves have not been able to stop Grendal and now are getting help from an outsider.
During the battles with Grendal and Grendal's mother there is a mood of suspense and energy. We are pretty sure we know Beowulf will win, but how? When? There is a moment when his men think he may have failed, and the mood is somber, until Beowulf returns from the battle in joyful triumph!
After the battles there is a mood of joyful celebration and of giving thanks for Beowulf's heroism. The party is light-hearted and the sentiments expressed between Beowulf and Hrothgar are heartfelt and sincere, which adds a dignity to the mood of the party.
At the end of the tale, at Beowulf's funeral, there is certainly a solemn mood over the people as they respectfully honor their fallen King and mourn his passing.