Death Fantasy Opera

At the castle, a sumptuous banquet is held every night. The participants are vampires, unicorns, and other deformed nobles. A beautiful human brother and sister wander into the castle and are forced to participate in the "game" of the banquet... The fate that awaits the losers is death as a sacrifice, or the humiliation of becoming a slave.

(Source: Kodansha, translated)

Note: the volume release includes the firsts 9 chapters of the manga. The rest of the serie never received an official release, and is now unavailable after the shutdown of the web-magazine e-youngmagazine on which it was published.