Atiesh was a summoned demonboss encountered in Stratholme as part of the [Atiesh, Greatstaff of the Guardian] quest line. Atiesh was summoned on "consecrated earth" in the Festival Lane, between Elder's Square and King's Square (between the Scarlet and Baron runs, in other words).
The demon appeared to dual wield two [Andonisus, Reaper of Souls]. He would drop one of these swords ONLY if he was disarmed during the encounter which a player could use against him.
He hit approximately 600-1000 with each hand on a Naxxramas-geared tank, and hit 1500 MH before dropping his weapon. He had over 200,000 HP.