The Ritual Knife is a Melee Weapon in Fallout 3. It was added in the fourth DLC pack called Point Lookout. It's a unique variation of the Knife that can be found at the Ritual Site during The Dark Heart of Blackhall quest.
This weapon only uses 12 Action Points per attack, making it the second lowest in the game. However with a damage stat of 6, it's practically useless to higher level characters.