Vritra

Vritra  is the evil Black Dragon and One of the 5 Dragon Kings. When Vritra was vanquished and sealed into Sacred Gears, its soul was divided into 4 parts Dragon Pulse, Evil Dragon's Black Flame, Dominion of Darkness, and Cage of the Dragon these 4 parts of Vritra's soul have different appearance and have been collected and sealed them in Saji's Body. Azazel hypothesized that due to contact with Issei, Vritra's consciousness appeared and might possibly unify all the different Sacred Gears becaouse the other sacred gears melted together and Vritra's consciousness was restored. Vritra possesses a wide range of special abilities. Though its power might be lowest amongst the dragon kings, Vritra is the king of techniques in terms of variety and unconventionality.