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.

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 the Movie: 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.