Six Knights of Black

The are a powerful group of warriors from the Demon Clan whose strength was used by Stigma as a benchmark for that of the Ten Commandments.

3,000 years ago
The Six Knights of Black are mentioned by a member of the Fairy Clan while commenting on the strength of the Ten Commandments. The Six Knights of Black were former contenders for the Commandments who chose to form their own group, instead becoming the elite rearguard of the Commandments. However, due to being lawless, the Demon King abandoned them.

The Six Knights of Black were part of a battle against four of the races in the Sky Temple, where Baruja as an Indura brought destruction. When the victory seemed certain for the Demon Clan, the "Great Oshiro" appeared and defeated the Indura, allowing the Goddess Clan to seal the Six Knights of Black, the Indura and other demons within the Egg Rock.

Prisoners of the Sky
When the seal of the Egg Rock fell apart after 3,000 years, the Six Knights of Black were released from their imprisonment. Dahaaka was the first one to escape, being faced by the Goddess Clan's descendants the Celestial Clan, and just before he can kill Zoria he was defeated by Meliodas. The other Knights were released the next day along with many Gray, Red, and Blue Demons. They start killing the Celestials to use their souls as sacrifices to free Baruja as well. However, the remaining Six Knights of Black were defeated by the Seven Deadly Sins. Bellion decides to sacrifice his companions' souls to finally release Baruja. Just before his demise at the hands of Meliodas, Bellion also gives his soul to the Indura.

Demon Realm arc
The Six Knights of Black seem to have replaced the Ten Commandments as the elite guard serving Zeldris, the new Demon King. However, unlike most other Demons, they retain a certain resentment towards the Goddesses and Humans who made war on them.

When Ioladio escorts the Four Knights of the Apocalypse and their allies to the Demon Realm, Tristan, Lancelot, Chion, and Isolde are immediately arrested by the Knights of Black who accuse them of being Holy Knights of the Kingdom of Camelot. When they discover Tristan's true identity, Dahaka and Derocchio become upset at simply being in the presence of a Nephilim.

Trivia

 * A black knight, or dark knight was a stock antagonist in Arthurian legend who tended to lack motivation, except as an obstacle to be overcome.
 * "Black Knight" was a nickname of the real-life order Knight Hospitaller, on account of the black mantles the order adorned themselves with.
 * The Six Knights of Black appeared in the movie The Seven Deadly Sins: Prisoners of the Sky, released in August 2018.
 * The group is considered canon as it was mentioned in the manga; however, member statuses are semi-canon or pseudocanon, as they debuted in a movie but were created by Nakaba Suzuki himself.
 * Unlike most high-ranking Demons who use their powers in the form of a Black Mark, the Knights of Black use it in the form of a Red Mark.