Vritra

Vritra is one of the Five Great Dragon Kings. He is the Evil Black Dragon known as the Prison Dragon.

Appearance
Vritra has the appearance of a black Eastern Dragon with violet eyes.

Personality
Despite being called an Evil Dragon, Vritra does not show any of his evil personality. In fact, he appears to have a wise and very polite attitude. Whether or not this has anything to do with his soul being split into several pieces remains to be seen.

History
Prior to the series, he was vanquished by Indra and his soul was split and sealed into multiple Sacred Gears, one of which would end up in Genshirou Saji. There appears to be more than four Vritra Sacred Gears which are held by others, but it is unclear if they also hold parts of his soul (like the evil part).

Birth of the Breast Dragon Emperor
Vritra was mentioned in Volume 4 when Ddraig explained to Issei about Ophis and the Five Great Dragon Kings.

The Heroic Oppai Dragon
In Volume 7, the Grigori, under Azazel's orders, implanted three Vritra Sacred Gears into Saji, restoring part of Vritra's consciousness.

In Volume 9, during the battle against the Hero Faction and Yasaka, Vritra (having regained consciousness) was shown to have taken up the role as a mentor to Saji, instructing him on how to use his powers like with Ddraig and Issei.

The Legend of Oppai Dragon and his Lively Companions
In Volume 14, Azazel explains that even though Vritra is called an Evil Black Dragon he is cute compared to the three brutal Evil Dragons. Vritra, along with Fafnir, helped to wake up Ddraig from his sleep during the attack from Euclid Lucifuge and Grendel from the Khaos Brigade.

Vritra was asked by the Two-Heavenly Dragons for help in Volume 17 to convince the past possessors of Divine Dividing to shed light on Issei's new ability. This limited Saji's ability to use his techniques, same with Issei, as the consciousness of their Dragons weren't present. Vritra, however, returned in a timely fashion and Saji was able to reach Balance Breaker.

Powers & Abilities
Immense Power: As one of the Dragon Kings, Vritra possesses high level dragon powers though is stated to be weakest amongst them along with being considered an Evil Dragon, but pales in comparison to the other Evil Dragons in this iniquity.

Dragon Flame: Like most Dragons, Vritra has the ability to breathe fire. Vritra's flames, however, are black in color. His flames also has cursed properties and even deadlier that it can kill and curse a soul eternally.

Curse: Vritra can somehow cast curse, as it mentioned by Saji that Vritra`s black flames has the ability to curse a person and supernatural being`s souls for eternity that be able to kill them.

Telepathy: Vritra can somehow telepathically speaks through people like Saji,Grendel,Fafnir and Ddraig.

Shadow Manipulation: Vritra has the ability to control shadows as shown in one of his Sacred Gears.

Technique Master: According to Azazel, Vritra is the weakest Dragon amongst the Five Great Dragon Kings in terms of raw power. However, his wide variety of techniques make up for his lack of power with versatility.

Trivia

 * Vritra is the only Dragon King who is also an "Evil Dragon".
 * In early vedic religion, Vritra means "the enveloper".