From the submitter:
(actually a trick - the hole in the ceiling is actually a box that is supported by the top of the tree which was cut flat)
Dammit. You had me going there...
Deviant Artist Midnight Angel put together an arm blade stylized after the ones used in the Assassin's Creed video game series. Using a drawer opening mechanism, a spring for a screen door, and a throwing knife, the weapon works surprisingly smoothly. The full instructions are on Angel's page, and you can see it in action below.