My necromancy t-shirt. The front says "reduce, reuse, reanimate" and the back "Necromancy: helping to reduce our dependency on the funerary-industrial complex."
Plus the grey camo cargo pants and motorcycle boots. Not really a costume, but the right theme anyway.
If I had a party to go to, I'd probably go as a scratching post... wrap myself up in rope, with a carpeted hat, and sprinkle catnip all over it.