The amulet of doomion is an item used in the later stages of the Underground Pass quest. It is obtained by killing the level 91 demon Doomion. It is one of the three amulets used in the quest. The other two are obtained by killing Doomion's comrades, Holthion and Othanian. Players can use their amulets to open a chest that contains Iban's shadow.
After the quest, the amulets cannot be obtained and have no other use.
For an exhaustive list of all known sources for this item, see here