The World Behind the Mirror is a realm that is accessible behind all mirrors. Once a person is in this world, he or she can touch various mirrors to see what is happening in the world that the mirrors on the other side exist in. At an unknown point in time, the Magic Mirror ends up here after being framed for killing King Leopold, and this is how he communicates through the Evil Queen's mirrors. The Mirror begins working on a portal to escape his prison, but he doesn't finish building it.
He remains alone until Emma Swan and Regina Mills arrive after the Evil Queen imprisons them within the mirror as well. The Dragon reveals there is one possible way to escape the realm through a portal that Regina's Magic Mirror, Sidney, was working on when he was trapped here. Despite his best efforts, the Dragon cannot not finish it, though he suggests that if the three of them work together, they may have better luck. Before they can accomplish much, the Queen speaks through the Dragon with his heart, taunting Emma and Regina, before revealing herself in one of the mirrors with Henry Mills. She then orders the Dragon to kill Emma and Regina, and he, unable to disobey her command, transforms into a gigantic dragon. As he rampages, Emma and Regina duck behind a pillar and determine that if they can get the Dragon to blast the mirror with his fire breath, it could break and release them from this world. Henry helps them by using Hephaestus' Hammer on the mirror's other side, breaking Emma and Regina free. As the Dragon's fire breath bursts through the mirror, Henry grabs the Dragon's heart to preserve it.