Ganondorf (from the Legend of Zelda: Ocarina of Time and The Wind Waker)
Add green face paint, and use regular armor from a knight or mage costume. Dye hair orange, and get a HUGE broad-bladed sword (plastic, of course).
Presenting the Great Evil King of Hyrule and Gerudo Valley, Ganondorf.
Note: If going for the Wind Waker version, add a pillow underneath armor to make it look like you're overweight. Remember your evil. Shout a threat after you say Trick or Treat or something.
Thanks to Mac for this evil idea!