Here's how to make an easy hidden spike similar to the blade used by Altair. All you need are a few things you probably already have in your house. If you like this, please subscribe. Once I get 50 subs I'll make a follow up instructable showing how to make a springloaded version with a wrist strap.
To make this hidden blade, you'll need a basic cap-style pen, a needle, and glue. I used superglue, but hotglue will also work.
Step 2: Disassemble the Pen
Take apart the pen and lay out the pieces. There should be four.
Step 3: Reassmble the Pen
Reverse the ink cartridge and slide it back into its holder. Reassmble the pen as usual. The ink cartridge should now slide easily in and out of the pen.
Step 4: Add the Spike
Glue the needle into the end of the ink cartridge and give it some time to dry.
Step 5: Finished!
The finished product easily passes as a normal pen, and the spike is ready to deploy at a moment's notice. Obviously, this is a different kind of hidden blade than in Assasin's Creed, but the principle is the same. Have fun, and be safe!
Again, if I can get 50 subscribers I'll show how to make this springloaded, wrist-mounted, and trigger activated.