Black lace can be used generously at a Halloween Horrors in Victorian London Party. Lamp covers, hurricane lamps, table covers, even furniture covers. Nothing says 'Halloween Horrors in Victorian London Party' like black lace!
The formality of your invites will depend largely on the formality of your event. For kicks, let's say this is a traditional Victorian tea party: in which case, you'll elect to send a handwritten note on a tea card, which is a notecard specially embossed with images of English roses, tea sets, and other lady-like paraphernalia.