Black Knights are Tier 6 creatures of the Necropolis (they are an alternative branch of the game. You can play as either Death Knights or Black Knights and switch between them for free).
The key feature of the Black Knights is that they are similar to those in Heroes 3, namely the ability to inflict a curse (weakness) upon hitting and a chance to deal double damage (although in the third game, this was implemented through the Falling Sword effect, rather than just a second attack animation, but I used what was available).