Phantom Mages appear on floors 6 and 7 of Yukiko's Castle in the Midnight Channel. After being defeated, they may drop Blue Quartz and a piece of Golden Cloth. Selling 6 Golden Cloth to Daidara will unlock the Iai Katana weapon for the protagonist, which is the best weapon available to him during the Yukiko's Castle dungeon.
Phantom Mages will appear on the fifth and sixth floors of Yukiko's Castle. If they are accompanied by a Positive King, they will cause it to summon Bronze Dice instead of Secret Bambinos. After being defeated, they may drop the Golden Cloth material. Selling 6 Golden Cloth at Daidara Metalworks will unlock the Iai Katana weapon for the protagonist.
Phantom Mages are Shadows appearing in the 1st and 2nd Stories of Evil Spirit Club, and are infamous for their Wind Corrosion skill which makes the party more vulnerable to Wind attacks - primarily their Magarula skill.
While not problematic if only one appears, they ambush the party at P-Spots at Story 1 in groups of 2, virtually guaranteeing a combination of Wind Corrosion and Magarula before the party can even act, leaving them weakened and almost prone to death by the next casting of Magarula. Several other enemies exist in lower floors that operate similarly, having a Corrosion and Ma-la skill corresponding to different elements, but they do not ambush the party as often as Phantom Mage pairs.