Location of Pain Skull

[Vilk]

Nobody know or is the rule just basic ie -1% at each fight? If this is the rule that makes the pain skull one of the worthless item of the game in term of efficiency, 100 fights with half mana to change it to dead skull is so vain, either you find it too late as it generally happen to me either it's worthless because the game is easier as you progress in any difficulty mode.