Recently, the PCs in my group were investigating an area in the undercity (Sharn, from Eberron) where they had fought some aberrations. They found a couple of hastily-abandoned rooms which included little besides a mirror on the wall and a crawlspace. Inside the latter, they found a strange-looking bag with splotches on it, which then turned out to be the skin of human-being who'd had all the stuffing sucked out of him (still had dessicated bones & organs inside). One caught a glimpse in the mirror of a grayish humanoid form leaning over those looking at the skin-bag, even though nobody could see it, and caressing the back of one's neck with a tentacle. It disappeared from the mirror as soon as he raised the alarm.
The next few minutes were spent trying to detect whatever might be in there with them, with little success and growing trepidation on the part of the explorers. The artificer tried to examine the mirror, at which point it went dead gray and for an instant showed a small eye in its center, which rapidly expanded to fill the entire mirror, which then exploded. Once the bleeding and dazed artificer was healed, he managed to infuse a
see invisibility spell, which revealed the two rooms to be full from roof to ceiling of invisible, intangible webs, which were moving very gently in a non-existent breeze. The party got the hell out of there