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 member of the Fairy Clan whilst 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.
 * The Six Black Knights will be are in the movie The Seven Deadly Sins the Movie: Prisoners of the Sky to release in August 2018.
 * Black Knight was a nickname of the real life order Knight Hospitaller, on account of the black mantle's the order adorned themselves with.