Complete every level on at least medium or harder. Once you return to the tavern, there should be a door above the northern (map view) staircase, with a big neon "OPEN" sign above it. Inside is a security camera, boombox (change your taverns BGM), a personal forge, and a single chest that only appears once. Inside the chest are heirloom weapons from every character's parents as well as 70k mana. Best of all, only the tavern only may enter this room.