I think I would actually faint away if I should open my door on Halloween night to someone dressed in this undead Witch lord costume.
Here's why I like this warlock costume: You get to wear comfy shoes, you can wear a simple tshirt and shorts underneath, and you don't necessarily need to gum up your face with a bunch of makeup to look scary.
Here's a modern take on the black magick warlock. A simple sexy coat worn open (please abstain for the love of god if you have an over hairy chest) and a pair of dress shoes. Add a snarl or come-hither look on your face, make weird motions with your hands from time to time, and you're good to go.
Here's a Medieval take on the warlock theme. Silver lines the bottom of the robe, the sleeves, and the inside of the hood. Add black or white gloves.