I am not a comp wiz so if anyone could list the specifics for the Pain Skull, and specially for the UPgrade of this item....
thanks in advance :)

+5 int +5 att, when the soul will be unbound. Lose 1%-3% random every battle so nearly 50 battles are required before you can "use it"

I think it depends on the strength of the army you are fighting. I had kept my Gremlin castle for this purpose but sometimes it didnt even go down 1% per fight.

Nope, it's random, fighting either weak or overpowering mobs didn't make one bit of difference for me.

Object's fight, like uipgrades, doesn't count for the skull. Don't ask me why

Because they don't count as actual battles. More precisely the skull drains your mana because that happens at the beginning of the battle when you're already inside but the percentage is reduced after the battle and there are no calls because there was no mob to begin with (item arenas are created on the spot).

is there a file in which it says what item is available at which shop?

yep, i do not knowwhicj tough, but save game scanner reads it

great, thanks! :)