Boss levels in Mega Man X6 are tied to the number of Nightmare Souls the player has obtained, up to a max of Level 4 at 5,000 Nightmare Souls. If Nightmare Zero is beaten at Level 4, Zero's black armor from previous games is unlocked. This armor is only normally unlockable with a cheat code entered at the title screen (L/L/L/ZR). Due to an oversight, the final stage unlocks automatically when the player obtains only 3,000 Nightmare Souls, meaning that Nightmare Zero is impossible to fight at Level 4 without a GameShark code (300CCF5F 0003 for the US version of the game) since he becomes unavailable when the final stage is unlocked.