I apologize beforehand if you find this instructables lacking, it is my first. In this instructables I will show you how to make an assassins creed hidden blade, not one based off of a game, just the concept that is both functional and sturdy. Don't be intimidated by the number of steps, this is a fun project that should only take part of a day if you have a dremel. With a hacksaw it will take a bit longer and the cuts won't be as neat. Mine was made with a hack saw and it turned out decently. This project cost me 20 dollars for my current setup, but if you take away some unnecessary things and account for parts you already own you may have to shell out 10 or 15 bucks. Because I made my blade before this instructables, I won't be able to post pictures of the process. But I'm sure you won't need to see me drill the holes or cut down the parts. I'll show you the parts afterwards .

My hidden blade is designed to be sturdy, functional, as pleasing to the eye as possible, cheap, easy to assemble and disassemble, and to require no special parts or tools. it is designed to be operated all on its own, with the only necessary motions be a jerk or swing of the arm, and extending the hand to retract. I first made a design that opened automatically instead of retracting, but it required you to use your other hand to retract it manually, which defeats the purpose. this blade is extended and retracted with a single arm. It is essentially perfect! But I'll let you decide that.

MATERIALS:
Drawer slide: should run you no more than 15 bucks, I had one lying around tho. Must be a side mounting one, not a bottom mounting one. They come in pairs of two, so if you got the stuff, make a pair!

Blade:
I used a bayonet from ww2 , 9 inches long. Its a bit too long, so if you can get a 7 inch blade it would be best.
Two small coil springs:
You can get a pair at home depot for 3 bucks, but I had some from a different project. Not too tight or you won't be able to eject the blade.

Flat spring:
For this I took apart a pants hanger. I used the metal clip and cut it down some so it had a nice curve upward. I'll show you pictures later, so bear with me! Its worth it!

4 machine screws with matching nuts. You're going to have to cut them down to different lengths so the starting length isn't important.

Thin rubber or plastic washer(optional). I had to use this because my blade was catching a bit. It just depends on your blade and slide.

Thin but strong string, about 8 inches. I used the windings of paracord, but now I use some steel cable.

A ring or a key ring. Must go over your ring finger easily

Two straps: I used cloth belts, but leather ones or Velcro would be an improvement.

TOOLS:
Dremel with cutting disks( optional. Alternative is hack saw, possibly tin snips but I don't know if they will work)

Drill and drill bits. The size of the bit needs to be the size of the screws you use.

Screw driver or needle nose pliers

Disclaimer: Please use common sense when making and wearing this prop. If you have any concerns about the blade material feel free to replace the steel blade with a wooden or acrylic analog. If you're planning to tackle this project then you most likely already have all the skills necessary to make whatever changes you deem necessary to make this prop suitable for your specific situation and needs. That being said, this may not be the easiest project to start, and may seem a bit difficult. if you can understand the mechanism then it is extremely simple. Be safe and be smart! I am not responsible for any personal injury or antipersonnel injury with this design. It is a real weapon and must be treated as such! Also keep in mind this weapon is illegal in most states. So don't be wearing it in public. Dulling the blade is a good idea ( I know, sad right?!)
I apologize for the lack of pictures, I will post them soon. Keep in mind, once you've made the parts, it is much easier to assemble at the end instead of as you go.

Step 1: Making the Slide

This part requires measurements. You're going to have to take your drawer slide out. Once you have it in hand, disassemble it. You're going to only need two tiers. The main one and the one right above it. So you should have a tier, a small track with a lot of ball bearings, and another tier. What you're gonna do is use your dremel or hack saw to cut down the main or outermost tier to your preferred length, wrist to a few inches below your elbow. Mine measured just under 9 inches, so that's a good mark to aim for.

Step 2: Second Tier and Blade

The second tier is a bit harder to make, but pretty simple. It probably has a lock on the bottom of it to keep the tier from flying out the end, so make sure you use that section for this part. I cut mine down to 3 inches long, but it depends on how long your blade is. If you have your slide at 9 inches, you probably don't want the combined length of the blade and second tier to be longer than 9.5 inches. Other than painting mine black and the little hole at the top, it will look like that. The hole comes later.

Now for the blade. Disassemble whatever knife you chose. Mine was a fixed blade bayonet, so I used a dremel to cut it from the handle. It didn't have any pre-existing holes, so I drilled one. Basically as long as it has a hole centered at the bottom of the blade it will work.

Step 3: Locking Mechanism

This part also requires some measuring. This part is the little metal piece I have a picture of. Basically what you want is a curved piece of metal that is able to stick up thru the big slots in the drawer slide. If yours doesn't have those. You can out it on the front of the slide, and dremel a catch into your blade. But to make it, I used a metal piece from a pants hanger, cut it in half, bent it upwards, and cut away at the sides to make it thinner. Set this too aside.

Step 4: Straps

This part is easy so you can take a nice deep breath. All you need is two belts that have the buckle on them, and cut the first one down so its a few inches longer than the circumference of your wrist. The second one you make a few inches longer than the circumference of your upper arm. Use the slide to determine positioning of straps. In the picture I have aa knife sheath connecting the two, it is not necessary. I put it on because I thought it made it look closer to kenways blade in blackflag.

Step 5: Drilling

This section may take a bit depending on your drill and skill. You're gonna take your bottom slide, the biggest tier. You're going to drill 3 holes. One at the front, to screw on the front strap, one a few inches below for the locking mechanism, and one near the bottom for the back strap. The middle hole is the hardest one. You need to position it so that it will catch on the second tier, will allow the slide to move completely forward, and will stop it far enough forward. If its too far forward, it won't catch the slide, too far back and it will allow the blade to fall too far backwards. So placement is everything. A after you've drilled the holes in the slide, you gotta drill one in the second tier. This hole will be for mounting the blade. Next hole goes in the base of the blade, assuming you don't already have one. Last hole goes in your locking piece of metal. If you don't have access to a drill, you could use hot glue to hold it all, which is how I did it at first, but it will break on you constantly. For a picture of how mine looked, refer to the picture in the first step.

Step 6: The Bolts

If you use a hot glue gun, you can skip this step. If you use the bolts, this step is very quick with a dremel. With a hack saw it would be hard. You have to measure how long you want each bolt to be. The first bolt will be used to hold the wrist strap to the slide. You don't want it to be too long because it will poke into your wrist. For this one, I put it down thru the slide, thru the hole in the strap, and then you put on the nut. You're going to tighten it almost as much as you can, then cut off the excess. Maybe cover it with a piece of felt then, up to you. Second bolt is for The rear strap. This one needs to be longer. It is what will keep your blade from flying out the back when retracted. This one you put thru the strap, then up thru the bottom of the slide so the excess of the bolt sticks up. Then you put the nut on and tighten down. The third bolt you use to attach the blade to the second tier. You put the bolt up thru Tue hole, slide on the rubber or plastic washer, slide on the knife, then the nut and proceed to tighten it down. Leave room on this bolt for a second nut . this bolt holds the springs. A good idea would be to drill a second hole thru both the second tier and the blade in order to keep the blade from turning sideways. I didn't do it because I had no room on mine. If you just tighten it very tight tho it will hold just fine. The last bolt goes down thru the middle hole, where you put on the metal locking piece up thru the bottom and then the nut. Tighten down.

Step 7: Almost Done! Spring Attachment

You're almost there! This last step, you take your dremel or hack saw, and you cut slits in the rear of the biggest slide, the first tier. Lateral slits, not very long. Stand the tier up on end with the front end on the ground, and cut downwards about half a centimeter on each side. Once done, slide the loop end of your springs into them, one in each. Put the other ends on the bolt that holds the blade to the slide. Take your last nut and put it on that bolt so it holds the springs in place. Now you have most of the mechanism. The last part is to tie one end of the string to the locking metal piece, and the other end to the ring. Depending on hand size, very the length.
*Revision* I have since updated how I attach the springs. If you look back to the drilling section and see the holes I drilled almost diagonally from the back of the slide, this is where those come in handy. It's also the source of the optional section of pictures in the introduction. you can do it the old way, but this way makes swapping springs easier to do. Once you ha e those holes drilled, take two more screws and put them up thru the holes, slide on a loop of the spring, and tighten the but on top. Simple as pie!

Step 8: Assembly

Assembly can be done in many different ways, but it is easiest to start with your locking mechanism. First you want to put one of your shorter screws down thru the top so that the head is sticking up. Go ahead and slide the spring or bent piece of metal on to the screw using a previously drilled hole, then put on and tighten the nut. Now you're going to bolt on the blade. stick a screw up thru the 3 inch section of the second tier, slide on the blade using the hole you drilled, then tighten down the nut. It is important that this nut is tight so that your blade doesn't go crooked. Next take your length of string with the key ring attached. You need to tie this around the bit of metal you use for a lock. I used a bow string that already had a loop on one end. you'll be pulling this to retract the blade so it needs to be sturdy. Now you get to add the front strap. You bolt it on just like the rest. Drop a screw thru the top hole on the main slide, slide on the strap thru a previously drilled hole, and tighten down a nut. the back strap is done almost the same, except this one you want to stick the screw upwards so that the threads come out the top. Which means you poke the screw thru the strap before you bolt it to the slide. Once thru the strap, stick it up thru the bottom hole and bolt on. You're almost done! you should have the lock, the blade, and the straps. All that is left is the springs. This part depends on your method of attaching the springs. If you're using the optional screws, refer to the pictures. If you're using the slits you cut, use some pliers to bend a part of the metal, slide the spring in the slit, and bend back the metal. Now to use the screws, put the screws up thru the bottom, slide the springs on, and bolt down. Last but not least, slide in the blade and attach the other end of the springs to the bolt on the blade. Now you have a hidden blade! Strap it on, put the ring on your finger, jerk your arm to extend the blade, and pull the string to retract it. Depending on your skills and materials, it should be sturdy enough for all of your assassin needs. I've tested the design on a ballistic gel I made that highly resembles the density and force needed to penetrate a human abdomen. My blade held up perfectly for 6 consecutive stabs, full force with a stab every second. The only thing that went wrong was that my knife blade twisted sideways. This is why I suggest adding a second screw. Enjoy! Post any questions and suggestions you may have! Happy hunting assassins!

Step 9: Optional Bracer

this step is optional, I just did it because I liked the look, and it made it more stable on my arm. This bracer is very easy, its just a soccer shin guard I picked up at good will for a dollar, and wrapped the straps around it and spray painted it black. this one has a strap close to where it would be on your upper calf, if it were worn, and it has a loop that goes around your ankle. That's the foamy part at the top. I tied the two upper straps together, slid it under the hidden blade straps, put it on, and tightened it down. It holds wonderfully and is extremely easy to add. I've also put some screws thru the bracer so that the straps don't slip around. Not a necessary thing, just convenient.