Will O' Wisp, also called "ignis fatus," "corpse candle," "fairy light," etc., is a famous spirit that appears in the form of light in forests, swamps, bogs and marshes at twilight or night. They appear as a flickering lamp, and this misleads them as being mistaken for Jack o' Lantern. It is also the name of a scientific phenomenon caused by the oxydation of Phosphine and Methane produced by organic decay that causes photon (light) emission. In the Irish folktale of Henry Glassie, Will O' Wisp is known as Willy the Whisp.
"One of the more famous names for the mysterious lights seen over swamps. They are the souls of those who fail to go to the afterlife and instead wander the wilderness, misleading travellers."
Will O' Wisp can be found in the Shinjuku Medical Center, the Shinjuku area of the world map, Shibuya and the Labyrinth of Amala's Second Kalpa. It sometimes drops Medicine, Life Stones and Beads when defeated in battle. As a member of the Foul race, it can generally only be contracted during when Kagutsuchi is full, the Demi-fiend has the Jive Talk skill, or if it is begging to be spared.
"The restless soul of a human being who wanders the earth instead of passing to the realm of the dead. They appear near lakes, graveyards, and other deserted places. They are said to lead travelers who encounter them down the wrong path. Will-o'-wisp are sometimes confused with Jack-o'-lanterns. Jack-o'-Lantern is said to have originated from will-o'-wisp."
—Shin Megami Tensei IMAGINE compendium
Will O' Wisp can be encountered in Nakano and appear as a regular tutorial mob.