The Moon Dial is a time travel device owned by Khromm. It was used in Portal Into Time for Thundarr and company to travel back in time to "Old Earth" and find a replacement part for the Guardian Machine at the Alamo. It was destroyed after Thundarr and Ariel retreived the part they needed.
One thing that's noteworthy is that the room holding the Moon Dial in Khromm's palace is actually part of the Moon Dial. The actual portal opened in the wall of the room, and the power source of the Moon Dial was seemingly located elsewhere in Khromm's Fortress.