Evil Pieces

The Evil Pieces, 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 using the crystals available in the Agreas Island 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, the Old Satan Faction rejected 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.

Evil Pieces are commonly given to Pure-blooded High-class Devils, so that they can gather servants of their own. Reincarnated Devils are also given the chance to receive Evil Pieces should they rise in rank and become a High-class or Ultimate-class Devil like Tannin and Rudiger Rosenkreutz​.

When a Devil receives the right to obtain the Evil Pieces they are required to go to the Satan's territory and touch a monument which functions as a replacement King Piece to register themselves as King.

If a King wishes to trade a Piece with another King, a ritual must be performed. A trading-purpose magic circle must be drawn. The Kings stand at the side of the magic circle while their pieces stand in the centre of it. The Kings increase their auras while putting their hands forward and activate the written Devil symbols to circulate. The auras of everyone around then begin to glow and synchronise, bypassing the magic circle. With that, the ritual will end and the trade has been complete in a simple manner.

Appearance
The Evil Pieces are identical to normal chess pieces in terms of appearance but will glow in accordance to the magic color of its users when 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 and Dragon God into Devils even with a Mutation Piece.

The Evil Pieces can only reincarnate those who died recently.

Pieces
The strengths, talents, and skills of the being to be reincarnated plays a large role in determining how many pieces will be used for reincarnation. Different Evil Pieces cannot be used together. Evil Pieces can also be exchanged between top-class Devils with peerage.

Queen
Queens are worth 8 Pawns. Queens possess all the characteristics of Rooks, Knights, and Bishops, making them the most balanced piece as well as the most powerful.

Rook
Rooks 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.

Bishop
Bishops 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.

Knight
Knights 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.

Pawn
Pawns 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).

Mutation Pieces
There are also special pieces known as the Mutation Pieces. 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.In Volume 23, Ajuka revealed to Issei that all his 8 Evil Pieces turned into Mutation Pieces.

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.

King
King  pieces are rare pieces created in the early days of the Rating Games. Unlike the other pieces which grant special abilities, the King Piece simply boosts a person's power. However, the King piece's strengthening is anywhere from 10 to 100 times and more. There are restrictions to its use. One, the user cannot already have an Evil Piece(s) because the King piece would overlap with their current piece(s) and expose them to dangers. Two, the user can potentially die if they are too strong. There are only nine unused King Pieces in existence, but they were banned by the current government from use for fear that Devils would use it to pursue evil desires. After Volume 21, the King Pieces has been confiscated.

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 Sacred Gears if they manifest themselves.
 * All known Rooks did not originally own a Sacred Gear.
 * All known female pawns have only consumed 1 pawn each.
 * All known Devils that were reincarnated with mutation pieces are male.