NOTE: "I don't like theme parties" is NOT a valid answer.

Mine:

SERIAL KILLERS.

I'm doing this for my birthday next month.
I'm dressing up as Mary Bell, and two of my housemates are going as Jack The Ripper and The Giggling Granny.

Ideally we should be serving Bloody Marys, but I don't want to, so I'm going to invent a drink with a suitable name. Any suggestions?

Soundtrack will include:
A lot of Nick Cave
A lot of Lupen Crook
The Cure - Subway Song/Killing an Arab
Mugison - What I'd Say In Your Funeral
Adam Green - I Wanna Die
System Of A Down - Chop Suey/Atwa/etc
The Adverts - Gary Gilmore's Eyes
Animal Alpha - Bundy
(lots more but can't be bothered to list everything. further suggestions are of course welcome.)

I might make an exception and say that it's ok to dress up as Fritzl as well.
Other than that, I hope people will show up looking like Jeffrey Dahmer, Myra Hindley, Micky&Mallory, Buffalo Bill, etc.