Spettrale Nudibranchia (Spectral Nudibranch, more commonly "sea slugs") is a Box Weapon consisting of several dozen sea slugs capable of casting illusions (usually flora), and can also become invisible. These illusions are real because of the Mist Flame's Construction Property. The sea slugs can also act like homing explosives when targeting the enemy directly. Genkishi can make himself invisible too, so that (when he is fully synchronized with the sea slugs) it is impossible to tell the difference between the two. In the second fight between Genkishi and Yamamoto, Takeshi used Rain Flames from his Rain Swallow to slow down the sea slugs, causing a difference in speed between Genkishi and the slugs so that he could attack Genkishi with relative ease.
- Danza Spettro Spada (Spectre Sword Dance): Combining his swordsmanship with his Box Weapon, Genkishi unleashes a barrage of sword strikes along with illusionary missiles. This attack can be dodged by moving at incredibly high speeds.
- Extra Danza Spettro Spada: Genkishi creates 9 copies of himself, which surround the enemy and each one uses Danzo Spettro Spada. The attack creates a globe of sword strikes and explosions, leaving no room for escape. Basil describes it 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.