Mirror House is a mobile auction house run by the Dealer.
They set up business in locations with a history of violence or atrocity, as part of a cult that worships evil as humanity's divine spark. It's used to sell super-villain artifacts and other instruments of evil to the highest bidder. Batman infiltrated the cult in disguise while investigating a series of murders, although he was quickly exposed and dosed with hallucinogenic drugs. Having barely escaped, Batman returned to fight the dealer in his airplane leaving Gotham. There was a massive explosion but no bodies were found, meaning the Dealer is probably still at large.