Illegal Move Triaina

Illegal Move Trident, also known as the Red Dragon Emperor's Three-Pronged Promotion, is a specialized technique used by Issei Hyoudou 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 Great Satans, Ajuka Beelzebub, creator of the Evil Pieces system, remodels Issei's Pawn piece after he learned that the original program written by him was overwritten by a different code, which he speculates to be the effects of the Juggernaut Drive, remodeling it to adapt to the power of the Red Dragon Emperor.

In Volume 9 while on the train on his way to Kyoto, Issei meets two of the previous holders of the Boosted Gear inside his conscious. One of the past holders, Elsha, then gives a box to Issei which she claims is one of Issei's future possibilities. During the battle with the Hero Faction in Kyoto in an attempt to rescue Kunou's mother Yasaka, Issei manages to awaken his new powers which allows him to use Promotion anytime and anywhere without the King's consent and changes his Scale Mail armor to different forms. He then uses this ability during his first fight with Cao Cao, who comments the three new forms of Issei's Scale Mail as "illegal moves" outside the Evil Pieces system. After Ddraig compares the three forms to Poseidon's signature weapon, Trident, 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. It also allows Issei to use Promotion without the consent of the King.

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 (Godspeed as he called it) that far surpasses Sairaorg's speed, but to make full use of it, Issei must shed his excess armor, causing it to become thin to the point where it has low defense, making him vulnerable to powerful attacks.

To change into Welsh Sonic Boost Knight, Issei shouts the form's name, in which he gains an increase in speed. He then shouts "Armor Release!" causing all but the absolute minimum armor required to fly to be shed in order to reduce weight and drag, further increasing speed. The armor then morphs into a more aerodynamic form, and announces "Change Sonic Booster!"

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 his 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, which he applies to the two cannons at the back of his armor. Its finishing move 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!"

Trivia

 * Welsh Blaster Bishop is modeled after the Gundam Double X, the titular Gundam from After War Gundam X.
 * The first form of Welsh Sonic Boost Knight is modeled after Wild Würger from Namco Bandai's Super Robot Wars series of video games, under Banpresto's Project ATX line of characters. The second form of Welsh Sonic Boost Knight modeled after the titular character in Kamen Rider Kabuto, the 35th installment of Toei's Kamen Rider Series. The two-stage transformation (Knight's first stage being its "Masked" Form and the second being its "Rider" Form) is also a reference to Kabuto's "Cast Off" function on their Masked Rider System.
 * Welsh Dragonic Rook is modeled after the titular mecha from Sunrise's anime series The Big O. Also, its finishing attack, "Solid Impact", is named after Big O's signature weapon, "Sudden Impact".
 * Unleashing the power of Illegal Move Trident has seemingly caused three of Issei's eight Pawn pieces to become Mutation Pieces, with them being of different values. Cardinal Crimson Promotion has transformed a fourth.