The Original Door is a door that transports the user off the Ark Arena and back to Earth; however it only opens when the Ark has travelled back 10,000 years, so it is only used by the victors of the Arena when they are transported back to the past to be humanity's progenitors. Opening the door before the Ark has finished travelling into the past has unfortunate circumstances for those who opened it, as shown in Chaos Rings Ω.
The Door transports the user to a cliff overlooking a jungle on prehistoric Earth. The Ark can be seen flying overhead, before it teleports back into the timestream.
In each campaign, the player uses the Original Door to tranpsort their characters to the past, with varying results. This will unlock the door to Bereshith Road, allowing the player to reach the True Ending of the story. The Original Door can still be selected at any time to view the events that occur if the characters go through it.
The Original Door opens after Vieg and Vahti defeat Cyllis and Dante & Rachel. It is The Almighty who opens it, according to Cyllis. However, the Ark had not finished travelling through time when the Door was opened, thus Vieg, Vahti and Ohm are deposited on the Ark's deck in a slowly collapsing timespace. This is where Cyllis fights them again. After the battle, Yorath transports them back inside the Ark and they use the Door again to return to Earth 10,000 years ago.