While the Danes are celebrating the death of Grendel, there is a tension that informs the reader that all is not well. There is a foreboding danger. Although Beowulf has defeated Grendel in a very important battle, there is a sense in the celebration feast that something more dangerous is about to happen:
At the feast, poetry is sung not only of Siegmund, a positive example, but also of Finn, who was supposed to be quite the opposite. Could this be foreshadowing that all is not well in Denmark?
Clearly, the most important battle is yet to happen. Grendel's mother comes to avenge her son's death. Grendel's mother carries off Hrothgar's friend and counselor. For this reason, Hrothgar is broken hearted. To kill Grendel's mother would be a most important battle because Hrothgar would be avenged of his close friend's death:
While the Danes are sleeping, Grendel's mother comes to avenge her son. She carries off Aeschere, Hrothgar's friend and counsellor, a man who had always stood at his side in battle. Beowulf finds Hrothgar broken with grief over the loss of his friend.
In a most important battle, Beowulf kills Grendel's mother. Now, Herot would is safe from all harm of monsters. No doubt, this is a most important battle for now the Danes can live in peace.
After Beowulf kills Grendel's mother, the Danes are rid of the horror associated with Grendel and his mother. When Beowulf kills Grendel, it is a major victory. Because Grendel's death brings about the presence of his mother, it is very important to kill Grendel's mother. With Grendel dead, his mother was still a threat to the Danes. No doubt, killing Grendel's mother was a major battle. Now the Danes are free from all harm.
The Danes can live in peace because Grendel and his mother are dead. When Beowulf killed Grendel, it made his mother angry. Grendels' mother was more of a threat than Grendel. Beowulf had a difficult task in killing Grendel's mother. Therefore, this battle was a most important one.