Grendel is a monster in Beowulf. He is a cursed individual and is haunted by the singing he hears from the mead hall and the warriors. When he is unable to hear the singing anymore, he attacks Heorot and the people inside. He continues this for twelve years until the mead hall is unusable. What makes Grendel so strong against the warriors in the mead hall is that he has a charm that protects him from any weapon:
All of Beowulf's band had jumped from their beds, ancestral swords raised and ready. Determined to protect their prince if they could. Their courage was great but all wasted, they could hack at Grendel from every side, trying to open a path for his evil soul, but their points could not hurt him. The sharpest and hardest iron could not scratch his skin, for that skin stained demon had bewitched all men's weapons, laid spells that blunted every mortal man's blade.
Grendel's mother is another cursed individual in Beowulf. She seeks revenge for Grendel's death. While Beowulf and Grendel's mother's fight is well matched, Beowulf ultimately wins and defeats her using a sword in her lair. The most fearful aspect of Grendel's mother is her outward demeanor and the distance to her lair. She lives under a large lake that takes Beowulf a day to reach. She is also a mother mourning her son:
She'd brooded on her loss, misery had brewed. In her heart, that female horror, Grendel's Mother, living in the murky cold lake assigned her since Cain had killed his only brother, slain his father's son with an angry sword.
The dragon is the third monster that Beowulf encounters in the poem. Beowulf encounters the dragon almost fifty years after Grendel and Grendel's mother. While Grendel and Grendel's mother are feared due to their size, appearance, and strength, the dragon is feared because of the destruction he has leveled on the Geats. Beowulf is only able to defeat the dragon at the cost of his own life:
So the king of the Geats raised his hand and struck hard at the enamelled scales, but scarcely cut through. The blade flashed and slashed yet the blow was far less powerful than the hard-pressed king had need of at that moment. The mound-keeper went into a spasm and spouted deadly flames, when he felt the stroke, battle-fire billowed and spewed.