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. 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 undeads. Players on Nightmare and Hell difficulties are likely to possess more useful amulets to wear, however.
Amulet of the Viper
Poison Resist +25%
+10 to Mana
+10 to Life