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 the strength 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 Commandments titles who ultimately chose to form their own group, becoming the very elite rearguard of the Ten Commandments. However, being so lawless, the Demon King abandoned them.

The Six Knights of Black were part of the battle against the four races in the Sky Temple, where an Indura brought destruction. When the victory of the Demon Clan seemed certain, 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.

Prisonners of the Sky
When the seal of the Egg Rock fell apart, The Six Knights of Black were released from their imprisonment. Dahaaka was the first one to escape, being faced by the Celestials and just before he can kill Zoria, he was defeated by Meliodas. The other five Knights were released the next day along with several Gray, Red and Blue Demons. They start to kill the Celestials and use their souls as sacrifices to free the Indura from the Egg Rock. However, all of the Six Knights of Black were defeated by the Seven Deadly Sins. Bellion decides to sacrifice his companions' souls to the Indura to finally release it. Just before his demise at the hands of Meliodas, Bellion also gives his soul to the Indura.

Trivia

 * A Black knight was a stock antagonist in Arthurian legend and tended to lack any motivation except as an obstacle to be overcome by honorable knights.
 * Black Knight was a nickname of the real-life order Knight Hospitaller, on account of the black mantle's 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, the members status is semi-canon, as they debuted in an original movie, but were created by Nakaba Suzuki himself.
 * Unlike most Demons who use the Power of Darkness in the form of a Black Mark, all Knights of Black used is in the form of a Red Mark.