Rumia is a human-eating youkai with the ability to manipulate darkness. She can be found around Hakurei Shrine and at the Misty Lake. Since she eats humans, it would be very dangerous for a human to come across her.

The darkness Rumia emanates will wipe out the light of torches. She can make her surroundings as dark as night even when it is broad daylight. However, she only uses her ability to manipulate darkness for support. She relies on physical force when attacking humans.

Since the darkness Rumia creates only envelops her surroundings, it just looks like a black sphere from outside of it. Apparently, this black sphere often bumps into trees while flying about, so Rumia probably cannot see that well in the dark either.

If you ever come across Rumia while strolling through Gensokyo, do not approach her. Since you are a human, she might eat you right up…

Rumia is one of the weakest youkai, so you should be fine as long as you keep a strong youkai around or have Reimu or Marisa with you. Reimu has actually fought Rumia in the past. Reimu had just left the shrine to go resolve an incident when Rumia bumped into her―Reimu beat the youkai easily.