RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Amulet of Othainian
The Old School RuneScape Wiki also has an article on Amulet of Othainian.

The Amulet of Othainian is a quest item obtained by killing the demon Othanian during the Underground Pass quest. When the player has collected this amulet, the Amulet of Holthion, and the Amulet of Doomion, the player can open a nearby chest containing Iban's shadow.