In the epic poem Beowulf, Beowulf engages in several battles with supernatural monsters: Grendel, Grendel's mother, and the dragon.
The battle against Grendel's mother contains an important instance of situational irony. Early in the poem Grendel's origins and ancestry are described by the poet:
Into a thousand forms of evil--spirits
And fiends, goblins, monsters, giants,
A brood forever opposing the Lord's
Will, and again and again defeated.
It is this reference to "giants" that is most interesting. Biblically, giants are said to have inhabited the earth in pre-history. This links the Beowulf story to Christianity and the Old Testament. But it also sets up a surprising development during the fight between Beowulf and Grendel's mother a little later in the story.
When Beowulf attacks Grendel's mother at the bottom of the mere, he finds the sword Hrunting (a gift from former rival Unferth) useless. Without a weapon he is in grave danger, having survived so far only because of his protective mail shirt. Then . . .
He saw, hanging on the wall, a heavy
Sword, hammered by giants, strong
And blessed with their magic, the best of all weapons
But so massive that no ordinary man could lift
Its carved and decorated length.
This is the weapon that saves Beowulf's life. Grendel's mother cannot overcome it's magical power and Beowulf uses it to decapitate her.
The irony here is that she is killed by a weapon created by her own evil ancestors. Beowulf was pretty much out of options at this point, and the unexpected appearance of a magic sword saves him at the last moment. We call this plot device "deus ex machina," which literally means "God from the machine." It refers to the last minute solutions and rescues we so often see in literature and movies. The ironic twist at the end is also still popular among modern readers and viewers.
Beowulf informs Hrothgar that if he should die in battle, the treasure Hrothgar has given him should go to Hygelac, Beowulf's uncle. Beowulf promises the sword, Hrunting, to Unferth if he should die.
Beowulf jumps into a large lake and is gripped by Grendel's mother. She can not hurt Beowulf because of his armor. But she manages to drag him to her lair (like a battle hall) while other monsters attack him while he is in the water.
He finds himself in her home, out of the water, away from the other monsters, and near a fire. He swings his sword at Grendel's mother but it has no effect: "this was the first time for the rare treasure that its glory failed." Beowulf throws down his sword, grabs Grendel's mother by the hair, and pulls her to the ground. She reciprocates, pulling him to the ground. She is on top of him and tries to use a dagger but it does not pierce his armor.
Among the armor in the lair/hall, Beowulf sees a "victory-blessed blade, an old sword made by the giants." He manages to stand up and grabs this sword. He swings, hitting Grendel's mother in the neck, and this kills her. Beowulf rejoices and spots Grendel's dead body. He cuts off his head. The men greet Beowulf and they bring the head back to Hrothgar.