Illegal Move Triaina

Illegal Move Trident, also known as the Red Dragon Emperor's Three-Forked Promotion, is a technique used by Issei Hyodo that changes his Scale Mail armor into three different forms based on the Evil Pieces system.

Summary
In Volume 8 of the light novels, one of the Four Satans, Ajuka Beelzebub, the creator of the Evil Pieces system, remodels Issei's Pawn piece so he can use Promotion anytime and anywhere, regardless if he has already done so.

In Volume 9 during his battle with the Hero Faction in Kyoto in an attempt to rescue Kuno's mother Yasaka, Issei uses this ability during his first fight with Sousou. Sousou makes a reference to Poseidon's signature weapon, Trident, and how it is similar to the three forms Issei assumed. Issei then decided to name his new ability "Illegal Move Trident".

Abilities
Illegal Move Trident has the ability to change Issei's Scale Mail armor into three different forms based on the Evil Pieces system, each with their own strengths and weaknesses.

Welsh Sonic Boost Knight
Welsh Sonic Boost Knight (龍星の騎士（ウェルシュ・ソニックブースト・ナイト） Werushu Sonikku Būsuto Naito), also known as the Dragon Star Knight, is Scale Mail's Knight form. In this form, Issei possesses enormous speed that far surpasses Sairaorg's speed, but makes his armor thin where it has low defense.

To change into Welsh Sonic Boost Knight, Issei shouts the form's name, after which the Boosted Gear announces "Change Star Knight!"

Welsh Dragonic Rook
Welsh Dragonic Rook (龍剛の戦車（ウェルシュ・ドラゴニック・ルーク） Werushu Doragonikku Rūku), also known as the Rigid Dragon Tank, is Scale Mail's Rook form. In this form, Issei possesses massive power and defense but has a low speed because of its massive armor. Its finishing move is "Solid Impact" (ソリッドインパクト Soriddo Inpakuto).

To change into Welsh Dragonic Rook, Issei shouts the form's name, after which the Boosted Gear announces "Change Solid Impact!"

Welsh Blaster Bishop
Welsh Blaster Bishop (龍牙の僧侶（ウェルシュ・ブラスター・ビショップ） Werushu Burasutā Bishoppu), also known as the Dragon Fang Monk is Scale Mail's Bishop form. In this form, Issei possesses an enormous amount of magical energy in which he applies it to the two cannons at the back of his armor. Its finishing attack is "Dragon Blaster" (ドラゴンブラスター Doragon Burasutā), where it fires an enormous amount of magical energy powerful enough to level an entire battlefield. However, it takes time to recharge once fired.

To change into Welsh Blaster Bishop, Issei shouts the form's name, after which the Boosted Gear announces "Change Fang Blast!"

Cardinal Crimson Promotion
Cardinal Crimson Promotion (真紅の赫龍帝（カーディナル・クリムゾン・プロモーション） Kādinaru Kurimuzon Puromōshon), also known as the True Crimson Dragon Emperor, is Scale Mail's Queen form that debuted in Volume 10 of the light novels during Rias's Rating Game battle against Sairaorg. Cardinal Crimson Promotion incorporates all three Evil Pieces traits, making it the most balanced form of Issei's Scale Mail. It is equally as powerful as Juggernaut Drive.

To use Cardinal Crimson Promotion, Issei says the following chant:

   
 * '''"I, the Red Dragon Emperor who is about to awaken
 * '''Who holds up the truth of the king up high
 * '''Holding the infinite hope and indestructible dream and walking the road of righteousness
 * '''I will become an Emperor of Crimson Dragon
 * And I will lead you to the path of Heaven, glowing in Deep(True)-Crimson light!"

After which the Boosted Gear announces "Cardinal Crimson Full Drive!"

Trivia

 * Issei's Bishop form of his Scale Mail is modeled after the Gundam Double X, the titular Gundam from After War Gundam X.
 * Issei's Knight form of his Scale Mail is modeled after Wild Würger from Namco Bandai's Super Robot Wars series of video games, under Banpresto's Project ATX line of characters. It is also modeled after the titular character in Kamen Rider Kabuto, the 35th installment of Toei's Kamen Rider Series.
 * Issei's Rook form of his Scale Mail is modeled after the titular robot in Sunrise's anime series The Big O.
 * Issei's Queen form of his Scale Mail is modeled after the ZGMF-X10A Freedom Gundam from Mobile Suit Gundam SEED.