Wiktionary
will-o'-wisp
n. (alternative form of will-o'-the-wisp English)
Wikipedia
Will-o'-wisp (Dungeons & Dragons)
In the Dungeons & Dragons fantasy roleplaying game the will-o'-wisp is a malevolent entity which makes its home in swamps and bogs. This aberration consists of a bizarre, spongy substance which hovers through telekinesis in the air, enveloped in a mysterious glow of yellow, white, green or blue.