Illegal Move Triaina, 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.
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 Evil Pieces, 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 subconscious. One of the past holders, Elsha, then gives a box to Issei she claims contains the delicate possibilities of the Red Dragon Emperor, which neither of them could reach. However, the item contained inside of it exited from Issei and started moving from place to place in Kyoto using the people who became arrested as molesters as a medium to increase its power through having them touch breasts.
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 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 Triaina".
Illegal Move Triaina 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 Rias' consent.
Welsh Sonic Boost Knight
Welsh Sonic Boost Knight (龍星の騎士, 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), 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, after which the Boosted Gear announces "Change Star Sonic!" (チェンジ・スター・ソニック, Chenji Sutā Sonikku)
Welsh Dragonic Rook
Welsh Dragonic Rook (龍剛の戦車, 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 body maneuvering speed because of the massive, thickened armor.
Its finishing move is Solid Impact (ソリッドインパクト Soriddo Inpakuto), a punch that has its power and speed increased by nailing the percussion hammer located at his elbow.
To change into Welsh Dragonic Rook, Issei shouts the form's name, in which the armor is enveloped in a dense aura which changes the armor's form into a larger form, after which the Boosted Gear announces "Change Solid Impact!" (チェンジ・ソリッド・インパクト,Chenji Soriddo Inpakuto)
Welsh Blaster Bishop
Welsh Blaster Bishop (龍牙の僧侶, 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 demonic power, 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), a severely boosted equivalent of his Dragon Shot capable of leveling battlefields and briefly warping space-time. The drawbacks to this is the amount of time it takes to charge the cannons and stamina lost with each shot. However, this can be replenished by Rias' Oppai Beam.
To change into Welsh Blaster Bishop, Issei shouts the form's name, in which a aura forms a backpack and two cannons, after which the Boosted Gear announces "Change Fang Blast!" (チェンジ・ファング・ブラスト, Chenji Fangu Burasuto).
- Triaina means "Trident" in reference to how the technique has three forms like a trident has three prongs.
- This would also be a reference to the technique's other name.
- Unleashing the power of Illegal Move Triaina 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. After Issei attained Diabolos Dragon, his remaining four Pawn pieces also transformed into Mutation Pieces.
- 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 16th 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".