Vritra's Sacred Gears

Vritra, the Black Dragon King (a.k.a the Prison Dragon), has multiple Sacred Gears that were created after his soul was split following his defeat. Four of these Sacred Gears are currently held by Genshirou Saji of the Student Council.

Summary
In Volume 3, Saji reveals that he owns one of Vritra's Sacred Gears, Absorption Line, which contains part of Vritra's soul in him.

In Volume 6, Saji reveals that a jewel appeared in his hand and gained many black snakes looking marks after absorbing Issei's blood during their master's Rating Game in Volume 5.

In Volume 7 of the Light Novels, Saji's abilities are upgraded when Azazel collects the remaining three Sacred Gears and implants them into Saji, thus awakening Vritra's consciousness. This gives Saji the new ability, "Vritra Promotion".

Absorption Line
Absorption Line (黒い龍脈（アブソーブション・ライン） Absōrpushon Rain), also known as the Black Dragon's Pulse, is the Sacred Gear residing in Saji. It looks like a small, cute lizard with a deformed face that appears on the user's hand. It has the ability to make lines of connections to absorb power from opponents. It can also connect the opponent to another object to transfer their power to that object, and the lines can also be used to swing around. In the Rating Game between Rias and Sona, Absorption Line was also able to drain blood from Issei.

Blaze Black Flare
Blaze Black Flare (邪龍の黒炎（ブレイズ・ブラック・フレア） Bureizu Burakku Furea), also known as the Evil Dragon's Black Flame, is one of Vritra's Sacred Gears that grants Saji the ability to unleash Vritra's deadly black flames to burn his opponents.

Delete Field
Delete Field (漆黒の領域（デリート・フィールド） Derīto Fīrudo), also known as the Domain of Darkness, is one of Vritra's Sacred Gears.

Shadow Prison
Shadow Prison (龍の牢獄（シャドウ・プリズン） Shadō Purizun), also known as the Prison of the Dragon, is one of Vritra's Sacred Gears that allows Saji to use Vritra's shadowy aura to restrain his opponents.

Vritra Promotion
Vritra Promotion (龍王変化（ヴリトラ・プロモーション）Vuritora Promōshon), also known as the Dragon King Transformation, allows Saji to take the form of the Black Dragon and use black flames. The flames can also restrain the movements of enemies, as well as absorb their power. This form allows Saji to use all of Vritra's abilities at unison.