Here's a look at a very detailed drawing of the one from the game: http://img.qj.net/uploads/articles_module/64010/assassin3_qjpreviewth.jpg
Originally, I got the plans from Wesley Scoggins on IndyMogul.com. I modified it a little, but kept the same basic structure/ provided more detail. Here's the website if you want it: http://www.indymogul.com/post/7272/wesleys-weekly-how-to-retractable-wrist-blade
Here's a video of it in action:
Let's get started!
Step 1: Supplies
3 Toilet Paper tubes-Free or $5 at Taco Bell
Pack of Paper Fasteners-$1.29
2 Rubber Bands
Can of Silver Spray Paint
Step 2: Making the Blade
Then, cut out your blade using your knife. Use a ruler to give it a nice, straight edge.
Take it outside and give it a nice coat of silver spray paint. Don't over do it though! You don't want any runs. I found that foam board dries surprisingly quickly, so by the time I got it back up to my dorm, it was already dry to touch. Once one side is dry, flip it over and do up the other side.
Now let it all dry for a bit while you work on the arm part.
Step 3: Working on the Arm Bracelet Thing
Set one aside and put the other two together.
Cut/poke a small slit through both rolls, about big enough for a brass fastener. Insert said fasteners.
Step 4: Putting the Blade to the Bracelet
Now, make two fastener sized slits towards the bottom of the blade, one closer to the bottom and one above it. Insert two fasteners and fold the top one down, but not the bottom.
Now, make another slit in the bottom of the tubes (in the center). Insert the unfolded fastener on the blade into the slit. This will hold it in place while you mess with it/attach rubber bands.
Next, take two rubber bands and string them from the two fasteners on the tubes to the upper fastener on the blade. The shorter/tighter they are, the better. Now take a piece of duct tape and tape the fastener on the blade so the rubber bands won't fall off it.
Step 5: Putting the Cover On and Finishing it Up
You may see in the pictures below that I didn't use a TP tube. This is because I didn't read the instructions fully and I didn't take more than two tubes from home (my college doesn't have tube TP, it's all paper core). That being said, the cardboard from a 12 pack of pop will work fine if you cut it to the size of a TP tube.
Use your handy duct tape and secure it. Cut a bit out of the bottom of the the 3rd tube so you can clamp the blade to the bottom tubes. Now take out the bottom fastener that held the blade in place and use the paper clamp to hold it there now.
Now you have a hidden blade!
The only thing to do now is secure it to your arm. Get a strip of duct tape, sticky side up, and tape from the inside of the device over your arm to the other side. Place another piece of tape on top to keep it from sticking to your arm/clothes.
Step 6: Tips and Advice
First off, you may want to reinforce the tip and upper section of the blade with duct tape. My tip broke about halfway through the day. It might be fine though if you're just going trick-or-treating, but if you're planning on wearing it a while/stabbing people with it, you'll want to reinforce it.
Buddah_123 suggested cutting some popsicle sticks and adding them to the edges of the blade to reinforce it. This seems like a good idea, and he says it works, so I'll trust him on this.
Second, if I were to do this again, I would use 12 pack cardboard so the bracelet part could be longer. This way, the blade could go further down the arm and be more hidden, compared to what I had with it sticking out on my palm. Also, use tight rubber bands. This will make a better spring and "pop out" effect.
Finally, this can be used with a string attached to the end of the clamp and running through the shirt to the other sleeve. A quick tug, and the blade is released. This takes a bit of practice, but it looks sweet.
I'm sure you can customize it more if you want. Also, if you want to stab people without it breaking (seeing how it's retractable and works like a prop blade) I would suggest some sturdier plastic (not too sturdy), shaping it, and painting it silver.
Overall, be safe, have fun, and enjoy!