I ran a session that turned into a bloodbath on Friday. The PCs, as agents of Department-7, were investigating a cult led by an illithid who had been trapped in the modern world. When they were getting ready to leave, the party's healer heard footsteps ahead... and they were supposed to be in an empty building (they had cleared the place out and locked all entrances). After that, the hacker saw a short humanoid figure with long, brown hair peeking around a corner ahead. She ran forward to engage the enemy and found herself face-to-face with six fourteen-year-old girls in school uniforms, all holding meat cleavers. Classic horror staple right there.
Immediately, the lights all went out and they heard light footsteps as the possessed schoolgirls charged. The girls' eyes glowed blue in the darkness...
My players were absolutely creeped out by the fact that there were no minis on the battlemat for the girls. They had no idea of where they were, or how many (there were a total of twelve, six having hidden in a side room). The PCs with flashlights were able to aim at those they were fighting, but they couldn't see whether or not more were lurking behind their current foe... or sneaking up to flank them. The girls focused on one PC at a time, attempting to surround that character and stabbing until he or she dropped, then moving on to the next target.
One of the things I kept mentioning that really unnerved my players was that the only sound the girls made was their footsteps. No evil laughter; no deep, low chanting. They didn't even scream when hit by a greatsword. Some of them even took several wounds before dropping.
What's gonna be even more fun is when I have the greatsword-wielding cowboy wake up at precisely midnight to find both pieces of the girl he sliced in two lying on the floor in his bedroom, with fresh blood and everything. Every once in a while I'll give him the impression that someone is sneaking up behind him, but nothing will ever happen. That should keep him freaked out for a good number of sessions.