What is the result of Grendel's attack in Beowulf?

Grendel attacks Hrothgar's mead-hall, Heorot, night after night for many years. In his initial attack, he slaughters thirty men. All the action of the poem stems from these attacks. Because of them, Beowulf travels from Geatland to help the Danes and pledges to fight and kill Grendel. The death of Grendel, the further attack by Grendel's mother, and her death at the hands of Beowulf are all directly caused by Grendel's initial attacks on the mead-hall.

It is only the events at the end of the poem, when Beowulf, fifty years later, must confront the dragon, which are not the result of Grendel's depredations. Presumably, however, the poet would not have recorded this abortive exploit of an aged king if the king in question had not already gained immortal renown by killing Grendel. Thus it is that Grendel, by attacking Heorot, gave rise to the entire poem.

Grendel is a savage monster, an outcast brooding alone in his cavernous lair night after night. His heart is filled with loathing and rage for the Danes, their happy, drunken laughter invading the night air as they feast at Heorot. But Grendel's going to bide his time; he's going to wait until all the Danish warriors are sleeping off their celebrations. So at night, Grendel suddenly bursts into the great mead-hall and starts brutally attacking the warriors as they sleep. In a matter of moments, he's killed thirty Danes and dragged their butchered carcasses back to the depths of his cave.

Night after night he returns, killing yet more Danish warriors and wreaking merry havoc on Hrothgar's kingdom. After twelve years of almost ceaseless carnage, Heorot presents a truly pathetic sight. It stands empty, its battered, blood-drenched walls no longer resounding to the strains of joyful song and merry laughter. And the effect of Grendel's savagery upon the Danes, as a whole, is disastrous. All of their finest warriors have been killed, meaning that there's no one who could possibly defend the kingdom from attack, whether it's from Grendel or anyone else. Morale is at rock-bottom; the kingdom lies vulnerable to destruction. What the Danes need is a hero. A hero who will bravely do battle with the vile, wicked Grendel and destroy this blood-thirsty monster once and for all. What they need is Beowulf.