Can't be chosen by Necrarch or Lahmian Vampires. The Black Axe of Krell is a Great Weapon and follows all the rules for normal Great Weapons. Also, any model which suffers 1 or more wounds from the Black Axe must roll a D6 at the start of both its own and its enemy's Magic phases for the rest of the game. On a roll of 1 or 2 it suffers 1 more wound with no armour save allowed (a Ward save can be taken as normal).