Six Knights of Black

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

Overview
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 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.

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 have a Black Mark, all Knights of Black have Red Marks.