Darth Knight has been using the body from a Dark Knight Movie Masters Batman figure, and the head, lightsaber, chest plate, and other pieces from a Star Wars Transformers Darth Vader action figure.
I transplanted the ears from the Batman head onto the Darth Vader head, and sculpted over them to blend them in with the sculpt of Vader's helmet. He features a paint job that really helps him stand out as his own concept, with silver accents, and other various parts painted in flat and gloss black.
He has extra Darth Vader pieces attached to his belt, his chest plate has been implanted into Batman's torso (and he still retains full ab crunch movement!), and his legs have been sculpted over with rectangular plates. He also features the classic Darth Vader waist cloak made from a sheet of vinyl. He comes with his lightsaber as well (or light baton if you wish for him to be as non-lethal as the real caped crusader). He's based on the costume design by Malmey Studios, but with my own unique flairs thrown in.
He's also up on eBay for anyone who might be interested. I won't post a direct link so as not to break any rules, but you can find him by searching for "Custom Darth Knight".
Thanks for looking!