Beowulf

Was Grendel a monstrous troll? Although scholarship about the epic sometimes assumes so, the manuscript itself does not describe him (this is Seamus Heaney reading his translation) that way:

Then a powerful demon, a prowler through the dark,
nursed a hard grievance. It harrowed him
to hear the din of the loud banquet
every day in the hall, the harp being struck
and the clear song of a skilled poet
telling with mastery of man's beginnings ...

(Beowulf, Seamus Heaney translation, lines 86-91)

That description doesn't sound like a troll, although when Grendel was sufficiently disturbed by the noise in Hrothgar's mead-hall, he attacked at night:

So, after nightfall, Grendel set out
for the lofty house, to see how the Ring-Danes
were settling into it after their drink,
and there he came upon them, a company of the best
asleep from their feasting, insensible to pain
and human sorrow. Suddenly then
the God-cursed brute was creating havoc:
greedy and grim, he grabbed thirty men
from their resting places and rushed to his lair,
flushed up and inflamed from the raid,
blundering back with the butchered corpses.

(Beowulf, Heaney translation, lines 115-125)

"Malignant by nature," the story tells us, Grendel "never showed remorse." In fact, he kept coming back, night after night, until no one in Hrothgar's great hall (which was similar to the mead-halls where Anglo-Saxons heard the tale) was safe from him. To avoid death, they had to avoid Heorot:

So Grendel ruled in defiance of right,
one against all, until the greatest house
in the world stood empty, a deserted wallstead.
For twelve winters, seasons of woe
the lord of the Shieldings suffering under
his load of sorrow; and so, before long,
the news was known over the whole world.

(Beowulf, Heaney translation, lines 144-150)

The king owned a building over which he had no nightly control. The people had a ruler who was, himself, ruled by a deadly monster. No one could stop Grendel. No one could kill him:

So Grendel waged his lonely war,
inflicting constant cruelties on the people,
atrocious hurt. He took over Heorot,
haunted the glittering hall after dark,
but the throne itself, the treasure-seat,
he was kept from approaching ...

(Beowulf, Heaney translation, lines 164-169)

Then, a warrior named Beowulf, heard about the deplorable situation. Unafraid of the monster, he had one objective in mind. Helping the Danes, he would fight Grendel.