Shadow Prison

Shadow Prison, also known as the Prison of the Dragon, is a Sacred Gear wielded by Genshirou Saji of the Student Council and the pawn of Sona Sitri.

It holds a part of Prison Dragon Vritra's soul and one out of the four Vritra-type Sacred Gears that Saji owns.

Summary
At some point, Grigori had previously retrieved three of the Vritra type Sacred Gears, including Shadow Prison.

It first appeared in Volume 7 of the Light Novels, when Grigori implanted it and other Vritra Sacred Gears into Saji during the final battle against Loki, where Saji used it in tandem with his other Sacred Gears to restrain Loki for Issei to strike him.

In Volume 17, Saji achieved his Balance Breaker as a result of the combination of the four Vritra Sacred Gears and the fusion of Saji and Vritra, all while he fought against Walburga.

Abilities
Shadow Prison gives the user the ability to use Vritra's shadowy aura to restrain opponents.

Malebolge Vritra Promotion
Malebolge Vritra Promotion ( , Māreboruje Vuritora Puromōshon ), also known as the Dragon King's Flame Prison of Guilt, is Saji's combined Balance Breaker of Shadow Prison with the rest of his Vritra's Sacred Gears which creates a Black Dragon Armor that strengthens the users powers and abilities.