Staff of the Archmage
Unique Item, discussed in this topic on UBB
You need to charge it in order to be able to use it - but how to charge is is not public knowledge (yet!).
To charge it you either need to spend 8/9 turns and/or about 65,000 mana charging. Therefore, it's difficult to make runs using it, as you will have to stop after each attack. It can be resisted like any other item. Battle sample below. Damage from Fire Elementalist is mixed in and not sure what was resisted but staff wasn't resisted or it would say so just like any item. Not sure if it has increased penetration.
(#xxxx) used Staff of the Archmage. Fire Elementalist casted "Arcfire". Fire Elementalist casted "Firestorm". (xxxx)'s 11900 Zombies are slain.
(via DukeRustfield May 26, 2008)