The Spectral Sea Slug is a Mist-attribute Box Weapon that constructs illusions with a Flame similar to mist. It appears to be 5cm big, but that is just an illusion and in fact is an aggregation of 8mm sea slugs. Many Mist illusionists conceal the true identity of their Box Weapons, and Genkishi is no exception.
Not only they can change their shape into anything, but each individual one possesses strong explosive powers. Their illusionary abilities were shown to have many uses, with the most blatant examples being invisibility and construction of a illusionary landscape. Illusions constructed with Mist Flames are "real", and those created by this Box Weapon are no exception. Also, with their capacity to fuse with illusions, the Spectral Sea Slugs can merge with their master by synchronizing their illusions. However, this combination can be easily broken if that synchrony is compromised, as Yamamoto proved by having Kojirou make the Spectral Sea Slugs become lethargic with its Rain Flames.
- Danza Spettro Spada (Spectral Sword Dance): by combining the powers of this Box Animal with that of the Spettrale Spada, Genkishi simultaneously releases a barrage of missile-shaped Spectral Sea Slugs and Mist Flame projectiles.
- Extra Danza Spettro Spada: a more powerful version of the Danza Spettro Spada, Genkishi creates a number of illusionary clones of himself so that he can release an even greater number of missiles and Mist Flame projectiles. This variation leaves no room for escape and is described by Basil as a solid mass of Flame energy.
- While the adjective "Spettrale" (like the English equivalent spectral) could be referring to either a spectrum or a spectre, the meaning in this case probably refers to a spectre, a reference to Genkishi's name's meaning, Phantom Knight.