Very little is known about this japanese monster, which is more often heard than seen. It is named for the distinctive sound it makes, crying out from the darkness: "Uwan!" No written record of its physical appearance exists, and the creature was thought to be utterly formless for centuries. It wasn’t until the Edo period that uwan were considered anything more than phantom sounds, when yokai artist Sawaki Sūshi gave the creature the shape we know today.

Uwan are occasionally encountered outside of old buildings and temples, where they like to assault lone passersby by leaping out of the shadows and shouting, "Uwan!" Any weak-willed individuals who faint at the surprise will never wake up again, as the uwan steals their soul and runs away into the darkness. However, if a brave individual shouts back, "Uwan!" this yokai will flee and never bother him or her again.

A famous uwan encounter took place in Akita prefecture during the Edo period:

A young newlywed couple had bought an old mansion and moved in together. On their first night in their new house, they were awoken suddenly by a loud voice shouting, "UWAN!" The shocked couple searched all over and around the house, but couldn’t find the source of the voice. The shouting continued sporadically all night long, every night for some time, and the couple was not able to sleep at all.

Some time later, the couple’s neighbors began to ask why he and she were always so tired-looking, with blood-shot eyes and disheveled hair. The husband tried to explain about the mysterious voice, but none of his neighbors claimed to have heard the shouting, and so of course, nobody in the neighborhood believed the couple. Instead, gossip quickly began to spread that the newlyweds weren’t getting any sleep on account of nocturnal activity of a different kind… Embarrassed, the couple ceased asking about the strange sounds.