The hidden blade that Altair uses to murder his victims in Assassin's Creed is a great costume piece, and if you don't mind working with metal then you should be able to use the instructions in this video to make a really cool one.
From the Creator:
1. A ball bearing slide (mine was 25 cm / 10 inch)
2. A hack saw or other tool to cut metal
3. A nut and a bolt
4. drill bits (one that's 1 or 2 mm for the spring holes, and one to cut a hole for a nut and a bolt. The size of the drill depends on the size of the bolt you are using)
5. Rivet + rivet gun
6. 2 springs (a long one, and a short one)
7. Solid copper wire (I used it to make the weird 'unlock' mechanism shape)
8. A ring (a keyring is the best solution)
9. A string
10. Soldering iron + soldering equipment
Additional files / information:
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