After being chained in the cave for a long time, Gremlin appears in the cave, then greeting Legion with his human name, Naitō and proceeds to release him. After being released, Legion begin to attack the humans. However, he encounters Beast, but escapes soon due to not being interested in Beast, seeing him as nothing more than trash. Legion later attacks a boy named Kenta Miyake, with Haruto being around. Kenta was promptly saved by Haruto. While Wizard battles Legion, Beast also joins the fray as he needs his "lunch". However, even the powers of two magicians are proven to be inadequate. Sensing and acknowledging Haruto's desire to protect and his willingness to put his life on the line to do so, Legion weakens Haruto enough in a fight and uses his halberd to open the gate to Haruto's Underworld. The forced entry into Haruto's Underworld leaves Haruto grievously injured from inside, which was made worse when Legion started breaking the Underworld apart. Beast then proceeds to enter Haruto's Underworld to save his rival.
While fighting Beast in Haruto's Underworld, Legion encounters WizarDragon, who join forces Beast and Beast Chimaera. Though Legion's body is very small compared to the two inner Phantoms, he overpowers the two with little effort. When Legion is about to finish off Beast Chimaera, WizarDragon steps aside to protect Beast Chimaera and Beast, at the cost of WizarDragon's life. Due to the loss of a subdued inner Phantom which serves as the well of mana, Haruto is now reduced to a mere human being and can no longer use magic.
Legion then proceeds to burst out from Haruto's Underworld, giving Haruto some time to get stronger. Legion then encounters Beast. Though, even when Beast transforms into Beast Hyper, he ends up being defeated when his leg gets restrained by Legion's halberd. Legion then once again meets Haruto at a beach. Even tough Haruto tries his best to fight the Phantom, Legion proves to be stronger than Haruto. However, after seeing Koyomi begins to collapse, due to having not much mana left, Haruto sheds tears. The tears then becomes a new ring: Infinity Ring. Haruto then use the ring to become Wizard Infinity Style. Wizard then summons WizarDragon into real word but not in its Phantom form, but as AxCalibur. Wizard Infinity then overpowered Legion, with his super speed and AxCalibur attacks. Wizard then used the Dragon Shining Rider Slash to end the Phantom's reign of terror.
- Gate: Naitō
- Episodes: 29-31
- Destroyed by: Kamen Rider Wizard Infinity Style's Dragon Shining
- Mythological Basis: The Legion
- Height: 228 cm
- Weight: 145 kg
- Legion is the only Phantom that can enter an Underworld. This ties in with what his namesake was famed for: going inside a host's body to destroy it from the inside out.
- He is the second Phantom to be named after an actual demon, with Beelzebub being the first.
- His human form is played by Mitsu Murata, who previously played Bishop in Kamen Rider Kiva and Douji in Kamen Rider Hibiki.
- Coincidentally, both Naitō and Bishop are named after chess pieces, as Naitō could be read as Knight, by omitting the long vowel sound at the end.