What details make the arrival of Grendel in the mead hall frightening?

Mead Hall is where everybody ate and slept. It was late at night as Grendel stalks the hall. Grendel was of course all monster-like. THere is detail about the sleepers and some about Grendel. I think it is the anticipation of the attack that fuels the fright.

I looked for my Beowulf book to get you a quote but couldn't find it. As I recall, Grendel is pretty pissed off about all the music and merrymaking. Loud neighbours can b so annoying! Grendel tears the door off the hall, revealing the sleeping Geats. He feels all demonic and happy at the thought of slaughtering them all. Grendel grabs one of the Geat warriors and mauls him, drinking his blood and tearing lumps off him with his teeth. That's pretty frightening I guess.

"Then his rage boiled over, he ripped open the mouth of the building, maddening for blood, pacing the length of the patterned floor with his loathsome tread, while a baleful light, flame more than light, flared from his eyes. He saw many in the mansion, sleeping, a ranked company of kinsmen and warriors quartered together. And his glee was demonic, picturing the mayhem: before morning he would rip life from limb and devour them, feed on their flesh; but his fate that night was due to change, his days of ravening had come to an end.

Mighty and canny, Hygelac's kinsman was keenly watching for the first move the monster would make. Nor did the creature keep him waiting, but struck suddenly and started in; he grabbed and mauled the man on the bench, bit into his bone-lappings, bolted down his blood and gorged on him in lumps, leaving the body utterly lifeless, eaten up hand and foot. Venturing closer, is talon was raised to attack Beowulf....."