Now a throwing knife isn't very well suited for beeing fired from a crossbow. The bottom heavy knifes are supposed to spin around in the throw, which requires a ton of practice. But to be shot straight from a crossbow requires a specifically designed guiding system.
The simple, but effective weapon presented in this video uses four flat aluminum rods as guides for the knife, which works quite well. Three layers of Thera Band Gold drive the blades far faster than any human beeing could throw them - and with serious accuracy.
There is room for enhancements, especially the looks can be much improved. But for the 8 hours I spent on it so far, I am fairly pleased.
As a bonus, the second "Zodiac sign" prize slingshot is presented, made from two different types of wood, and featuring twin forks.