Evil Pieces

The Evil Pieces (悪魔の駒（イーヴィル・ピース） Īviru Pīsu), also known as the Devil's Pieces, are a set of 15 chess pieces given to top-class Devils to increase their ranks by reincarnating other beings into Devils.

Summary
The Evil Pieces were created by Ajuka Beelzebub to help replenish the number of Devils after the Great War which had caused the death of countless Devils. While many Devils were able to adapt to the Evil Pieces, only the Old Satan Faction of the Khaos Brigade resisted in the usage of the Evil Pieces, believing them to be a disgrace to Devils for reincarnating Humans and other races into Devils. The creation of the Evil Pieces eventually lead to the creation of the Rating Game.

Appearance
The Evil Pieces are a set of red, crystal-like chess pieces that glow when they are in use.

Abilities
Based on the game of chess, there are a total of 15 pieces (1 Queen ♛,  2 Rooks ♜,   2 Bishops ♝,  2 Knights ♞, and 8 Pawns ♟) that are given to top-class Devils (King ♚) with a peerage. These pieces are used to reincarnate other beings into Devils and become servants of the top-class Devil. The reincarnation abilities of the Evil Pieces, however, are not absolute as they cannot reincarnate Gods/Buddhas or Dragon Gods into Devils even with a Mutation Piece.

Pieces
Each piece has a corresponding value. These values are used to determine which piece may be used to reincarnate someone, their strengths, talents, and skills play a large role in determining their value. Different Evil Pieces cannot be used together. Evil Pieces can also be exchanged between top-class Devils with peerage.

King  ♚
High-Class Devils with a peerage are given the King Piece (王 Ō). The gender of the King makes no difference, as with the case of Rias and Sona.

Known Kings

 * Rias Gremory
 * Sona Sitri
 * Sairaorg Bael
 * Sirzechs Lucifer
 * Riser Phenex
 * Lady Phenex
 * Diehauser Belial
 * Diodora Astaroth
 * Tannin
 * Ruval Phenex
 * Roygun Belphegor
 * Bedeze Abaddon
 * Rudiger Rosenkreutz​
 * Ajuka Beelzebub
 * Serafall Leviathan
 * Falbium Asmodeus

Queen  ♛
Queens (女王 Kuīn) are worth 9 Pawns. Queens possess all the characteristics of Rooks, Knights, and Bishops, making them the most balanced piece as well as the most powerful. So far in the series, all of the Queen pieces have been female, with the exception of Tannin.

Known Queens

 * Akeno Himejima
 * Grayfia Lucifuge
 * Kuisha Abaddon
 * Yubelluna
 * Tsubaki Shinra
 * Tannin

Rook  ♜
Rooks (戦車 Rūku) are worth 5 Pawns. Rooks gain superhuman strength, leading to high offense and defense. However, they are not very quick, and can get beaten easily by a high-speed opponent. It was mentioned in Volume 2 and shown by Rias in Volume 4 that the King can perform castling with a Rook piece.

Known Rooks

 * Koneko Toujou
 * Rossweisse
 * Tsubasa Yura
 * Xuelan
 * Isabela
 * Ladora Buné
 * Gandoma Balam
 * Surtr Second (Mutation Piece)
 * Bahamut
 * Loup Garou

Bishop  ♝
Bishops (僧侶 Bishoppu) are worth 3 Pawns. Bishops gain enhanced magical abilities which they can use to conjure up a multitude of spells (offensive, healing, etc.). However, more powerful spells consume most of their magical power, which takes a long time to regain, forcing them to use their power carefully or risk becoming vulnerable to physical attacks.

Known Bishops

 * Asia Argento
 * Gasper Vladi (Mutation Piece)
 * Ravel Phenex
 * Momo Hanakai
 * Coriana Andrealphus
 * Reya Kusaka
 * Misteeta Sabnock
 * MacGregor Mathers (2 Pieces)

Former Bishops

 * Kuroka (2 Pieces)

Knight  ♞
Knights (騎士 Naito) are worth 3 Pawns. Knights gain increased speed and mobility, enabling them to perform high-speed attacks and maneuvers. However, their main weakness is their low defense, making them vulnerable to powerful attacks if they are not careful. Another weakness among knights are their legs; if their legs are injured, their mobility is greatly reduced.

Known Knights

 * Yuuto Kiba
 * Xenovia
 * Souji Okita (2 Pieces)
 * Tomoe Meguri
 * Karlamine
 * Siris
 * Liban Crocell
 * Beruka Furcas
 * Bennia

Pawn  ♟
Pawns (兵士 Pōn) are worth 1 Pawn each. The traits of Pawns is the ability to promote into a Queen, Rook, Knight, or Bishop in enemy territory (as seen in the anime when Issei enters the church to save Asia) or with the permission of their King (as seen when Issei asked Rias for permission to promote before fighting Riser to break off the engagement; however, this method cannot be used in Rating Games).

Known Pawns

 * Issei Hyoudou (8 Pieces, 4 out of 8 became Mutation Pieces as of Volume 12)
 * Mira
 * Ile and Nel
 * Ni and Li
 * Shuriya
 * Marion
 * Bürent
 * Genshirou Saji (4 Pieces)
 * Ruruko Nimura
 * Regulus (7 Pieces)
 * Enku (? Pieces)
 * Beowulf (? Pieces)

Mutation Pieces
There are also special pieces known as the Mutation Pieces (変異の駒（ミューテーション・ピース） Myūtēshon Pīsu). If reviving a person requires more than one piece, then using one Mutation Piece would be sufficient in reincarnating that person instead of having to use several pieces. Mutation Pieces can be a Queen, Rook, Knight, Bishop, or Pawn. They are said to be very rare, and only 1 out of 10 Devils possess the Piece. Normal Evil Pieces can also change into Mutation Pieces, as shown in Volume 12 when 4 of Issei's 8 Pawn Pieces transformed into Mutation Pieces, each with different values.

Unused Pieces
Another feature installed in the Evil Pieces are the Unused Evil Pieces, which will undergo changes according to the master's growth, allowing the King to use less pieces when reincarnating other beings into Devils.

Trivia

 * When Issei's body was destroyed and his Evil Pieces were summoned away, Issei was able to temporarily channel his sword Ascalon through one of them, causing it to assume the form of the sword, allowing him to lend it to Yuuto Kiba.
 * Ajuka Beelzebub has stated that there are many hidden secrets to the Evil Pieces but that he prefers to let the users figure them out for themselves.
 * The Evil Pieces can even reincarnate the spirits of Longinus if they manifest themselves.