Some items have hidden secondary stats. They show up in game as ??? but actually provide stats. So far they are:
Memento: Makes the Lich fight easier (Lich will only have 3/4 of its max health). A message box will also appears at the start of the fight.
Reckoning Wand: Seems to have a random chance to activate the Star Chunk-effect, which summons a wave of comets with decent damage.
Legend gear appears to have hidden stats, however this third stat is unlocked once the gear is upgraded to level 10.