Finally i could upgrade my pain skull to dead skull (for those who dont know this item: you have to wear this artifact for around 50 battles and it halves your mana.. it's really a pain
. but the reward is that after upgrading this artifact it will give +5 attack +5 int. My stats at lvl 20 : 24 att., 15 def, 32 int
Unfortunately it doesnt increase the phantom spell duration at 30 int :/. i dont know if its a bug or what. slow/haste/etc get a duration bonus.