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.