鬼火
One of the more dangerous types of hi no tama yokai, onibi is a beautiful but deadly phenomenon. Its name means “demon fire,” and it certainly earns that moniker. It look likes a small ball of flame, usually blue or blue-white (red and yellow onibi are less common), and often appears in small groups of twenty to thirty orbs. The orbs can range in size from three to thirty centimeters, and usually float around at eye-level. They appear most often during the spring and summer months, and particularly on rainy days. They appear more frequently in places that are surrounded by nature.
Onibi can be found all over Japan. In some areas, they are said to occasionally manifest the faces and even voices of the victims whose life force they have drained. In Okinawa, onibi is said to take the shape of a small bird.
Read more: http://yokai.com/onibi/