Time Machine

The Time Machine -- a mysterious device of unknown origin, capable of bending time and space to send its user back to the past or into the future.

This curious machine was needed in Ultima I to breach Mondain's fortress, which he had shifted to the Time of Legends. Once a certain princess had been freed, she pointed the Stranger to the location of the device. However, four colored gems were also needed to run the machine.

Once these were acquired and placed within the time machine's control panel, it instantly activated, sending the Stranger straight to the Time of Legends and inside Mondain's fortress for the final battle.

When Mondain was defeated, his fortress crashed back into the present. The Time Machine did not return, however, and thus it was ultimately lost -- along with any way of determining when/where it came from, and who created it.

Trivia

 * The Time Machine is one of the many anachronisms of the early games. It can be speculated that the Time Lord created it for the Stranger to use.