Andonisus was a legendary sword and dropped off Atiesh, a summoned demon in Stratholme which was part of the [Atiesh, Greatstaff of the Guardian] quest line. Atiesh dual wielded this weapon at the beginning of the fight and in the course of the battle dropped one sword which a player could then use against him.
Notes
Disappeared if taken out of Stratholme or after 10 minutes.