Amulet of the Viper

The Amulet of the Viper is headpiece that is transmuted with the Staff of Kings to create a Horadric Staff, as part of The Horadric Staff quest. This item drops from the Tainted Sun Altar on the second level of the Claw Viper Temple.

As a quest item, you can wear it on your Amulet slot. Although the poison resist, combined with a minor boost in life and mana, may make it an attractive choice for the player while he/she explores Tal Rasha's tombs on difficulty, which contain many poisonous undead, it will vanish when the quest is complete and cannot be picked up again on the same difficulty level.

Trivia
This seems to be the only amulet category item that is not an amulet. It's a unique item type(Amulet of the Viper) that's categorized as an amulet.
 * This is similar to how there are dozens of kinds of melee weapons. All other amulets are apparently the same base item with stats added on them.
 * Notably amulets have no actual stats as a base item. So for gameplay purposes this item is probably different to enable the cube recipe to work and prevent this item from being transmuted in recipes that use normal amulets.
 * Also regular amulets have a randomized appearance. The "Amulet of the Viper" item type has a different appearance that's static.