Time Machine

A Time Machine is what the name says, a machine created to travel to the past or the future.

A Time 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 machine. However, additionally four colored gems were needed to run the machine.

Once that was done, the machine activated, sending the Stranger right to the Time of Legends into Mondain's fortress for the final battle. When Mondain died, his fortress crashed back into the present, but the Time Machine didn't return, meaning that it became lost for all times.

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.