The Last Passenger was a title given to the mysterious, unnamed VIP aboard the Cyre 1313 lightning rail that hurtled forever through the Demiplane of Dread and the unwitting darklord of that domain.
On the world of Eberron, when the Day of Mourning struck the nation of Cyre on 20 Olarune, 994 YK, in the capital of Metrol, citizens crowded aboard Cyre 1313 to evacuate the city. However, the unknown, late-arriving VIP ordered a critical delay, even forcing hundreds of people off so they and their retinue could board in secrecy. Cyre 1313 finally departed, but too late, and was overtaken by the dead-gray mist and lost.
Only it wasn't destroyed. Cyre 1313, the Last Passenger, and all others aboard were spirited away, not by the dead-gray mists but by the Mists of the Demiplane of Dread. Thereafter, it ran ceaselessly through the plane and its Mists as a mobile domain of its own, with the VIP occupying the front carriage as its darklord.