Cross-World Mixology: Bartending For Another World

Every few days somewhere in another world a bar opens up for 3 days and nights only and disappears on the 4th day. The owner and his apprentice makes drinks for any race whether you're a king, a monster, the undead, or anyone else and to anyone who accepts hospitality to be in the bar of otherworldly tastes, a bar named the Phantom Cat. Some say the owner has a secret that his own people don't know except his regulars. What secret could it be?