In the world of Thundarr the Barbarian, time travel is not unheard of, though it is rare. Some powerful wizards such as the wizard Vashtarr are able to rip open the fabric of time using only their magic. Others, like the wizard Krom, rely on devices and machines to open a portal into time. The Moondial is one such device.
The Moondial apparently runs on magical energy. (If magic is not used in your campaign, the energy meeded to power this should be astronomical - such as a dedicated power plant or fission chamber.) Once enough arcane energies are poured into the device, it will activate - gears and knobs turning, dials spinning, and other lights and diodes blinking and flashing. Behind The Moondial is a large set of doors which will then slide open, revealing an image of the destination. This "time door" is the gateway to whatever time and place The Moondial has been set for. Once something passes through the time door, a portal is effectively "locked open" at the destination coordinates usable only by the original travellers. When the open portal is re-entered, the travellers will return moments after they left. For example, if Thundarr entered the time door and appeared in 1982, there would be a time door back to 3994 - his home time - at the same location he appeared at. Only he can see and re-enter the doorway back to his home time. (This is to keep others from wandering into the shining doorway to find themselves surrounded by wizards and moks.)
Setting The Moondial for a specific time and place is a very complex process understandable only by the most cunning and educated of wizards. Most likely, the PCs will use The Moondial to go back in time to whatever place and coordinates the device is already set for. Fooling around with The Moondial's settings is dangerous indeed. Anyone who tries to reset it should find themselves thrown to some random location in space/time. (If you really wish to teach them a lesson, have the return time door be inaccessible or missing.)
The wizard Krom uses The Moondial to loot the past, bringing powerful Ancient artifacts and weapons back to the future for his use. Fortunately, Krom is not a very imaginative wizard, settling on bringing back rifles, lasers, and armored tanks rather than more destructive weapons or more dangerous substances. Thundarr and his companions used The Moondial to travel back to 1982 to secure a replacement part for The Guardian. Upon their return to the future, Thundarr destroyed The Moondial so Krom would be unable to gain more weapons or - worse - fool around with and change the past.
A clever Mutant Lord should be able to find all kinds of uses and plots for The Moondial, but it should be used sparingly and with caution. After all, if Krom thought to bring working Ancient weapons and devices back to the future, what's to keep a clever PC from doing the same?
NOTE: This device was inspired by the episode "Portal Into Time" from the classic Ruby Spears post-apocalyptic cartoon"Thundarr the Barbarian." Stay tuned each week for Thundarr Thursday!