Here is the full list of all 15 Black Knight - there are Milena achievements.
Reached Black Knight Rank D
Reached Black Knight Rank C
Reached Black Knight Rank B
Reached Black Knight Rank A
Defeated the Orc Lord
Defeated the Forest King
Defeated the Troll Warlord
Rescued all the captured Gladiators
Went to jail with Celia
Defeated the White Reaper
Defeated the Harpy Queen
Defeated Quatro, for good
Defeated the Rabi King
Defeated the King Slime
Unlocked all pictures in the Gallery