The Athame is a silver dagger. Against most mobs or other players, it does ½ heart of damage, but deals a lot of bonus damage to Endermen. The Athame is capable of killing an Enderman in just 2 non-critical hits. Its item ID is 1329. The Athame has a durability of 72.
It is also enchanted very well; one level can give you a level 2 enchantment. However, due to its low damage rating, it's not recommended to spend experience on the Athame unless you are specifically hunting Endermen for the Ender Pearls.