Boroboroton

Origin
The Boroboroton is a yōkai from Japanese folklore. It was described by Toriyama Sekien as a a tattered futon (a Japanese nap mat) who comes to life at night. It rises up into the air and throws its (former) owner out of bed, then begins to twine around the head and neck of the sleeper with the intent of strangling him.

Effects
Activates when slept on. The artifact will begin to violently buck until the sleeper is thrown off, whereupon it will slide over the dazed victim and smother them with impressive force.

Kept safely rolled up on a shelf, it seems entirely passive as long as no-one tries to lie on it.