Ouch!
I had a rather memorable experience myself with the Skull of Pain to Skull of Death upgrade. I got the third item of the Set of the Dead, the Skull of Pain, and decided that there were enough battles left in the game to upgrade it to Skull of Death and thus complete the set. And there was, sort of. I got it upgraded after the battle with K'Tahu. So i had the pleasure of using it exactly once, in the battle against Baal
I think it needs 66 battles to be upgraded, or something like that.