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 normal 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.
Amulet of the Viper
Poison Resist +25%
+10 to Mana
+10 to Life