If you're really into Assassin's Creed, you should probably stick to just replaying the game or collecting figurines. Because buying a replica "extension knife" is a great way to have everyone think you're an unstable psycho.
Seriously guys, you don't want to be caught carrying around a knife designed to be hidden in your sleeve for covert murdering. No one will care that it's based on a video game. If you had an assault rifle from Modern Warfare 2, people would still focus on the whole "deadly weapon" aspect over the "fanboy" aspect, and the same will go for this. So just stay away. [Museum Replicas via Kotaku]