The Athame is a silver dagger. Against most mobs or other players, it does 1/2 hearts of damage, but deals a lot of bonus damage to Endermen. The Athame is capable of killing an Enderman in just two non-critical hits. Its item ID is 1329. The Athame has a durability of 72.
An Athame can be enchanted with sharpness or other enchantments to deal additional damage or effects.
It is also very good when in an Enchanting Table; one level can give you a level 2 enchantment. However, due to its low damage rating it's not recommended to spend XP on the Athame unless you are specifically hunting Endermen for the Ender Pearls.