The Staff of the Archmage is one of the most powerful staffs in game, one of which is a pinnacle of achievement for mages. Since only Mages can wield it, it rewards a bonus 100,000 XP for wielding it. Another thing that is awesome about is it high Special Attack bonus.
The Staff of the Archmage is only available by defeating Kathryn, Archmage of the Violet. It is also buyable from other players, but it is very rare. The last way of obtaining it would be to kill someone in the Nether Realm FFA that is wielding it.
The chances of getting it from Kathryn is 2%.