This is an arm blade I fashioned after the arm-blade seen in "Assassin's Creed." It's a bit complex, but if you're not afraid to get messy and make some mistakes then check the link below for detailed build instructions. Be safe and have fun.Midnight Angel posted a series of diagrams detailing the project on Deviant Art, but he warns that it is harder to make than it looks, but if you are up to the challenge, he offers detailed instructions on how to make one for yourself – though I recommend leaving it to the professionals. What you see in the video below is just a prototype, and he says that the forward locking system is not as strong as it could be.
Keep in mind that this more than likely constitutes a concealed weapon anywhere in the United States, so it's probably a bad idea to actually make one of these.