Sundial Slipway, Mercury
The Guardian activates the Sundial and is teleported to the Corridors of Time.
Corridors of Time
Ghost: The Shotgun you crafted in the Infinite Forge is reacting to the Sundial! An onboard transponder is broadcasting coordinates: a path through the Sundial, crossing two time periods. The prophecy blueprint you used to create the Perfect Paradox must have included this broadcast. If you can open up the initial chamber, I can align us to the first time period the broadcast is referencing.
The Guardian traverses the Corridors of Time, defeating the Vex which they encounter. The Guardian enters a portal to Mercury's past.
Zephyr Station Perimeter, Mercury
The Guardian finds Skiffs deploying Fallen units to fight the Vex.
Ghost: This is new… Topographical scan confirms this is Mercury circa the Dark Age. Incoming signal is an old emergency band… Hold on…
Saint-14: I repeat, this is Saint-14. The Fallen have overrun Zephyr Station. If you can hear this, turn back!
Ghost: Saint, hold your position. You have an armed Guardian incoming!
Saint-14: Who is this?
Ghost: Just stay alive, Titan.
The Guardian defeats the Vex and the Fallen. A barrier blocking a nearby corridor disappears. The Guardian searches for Saint-14.
Saint-14: All is lost, Guardian. Get out of here. I'll hold them off for as long as I can. The Fallen cannot be stopped. They do not negotiate. Their bargains are lies. I've watched them burn and pillage whole villages in the Cosmodrome. [gags] I've watched Dregs eat children. They envy us. Their legends say the Traveler chose them before us. And they will kill. They will torture. They will maim to earn their Machine God's favor.
The Guardian defeats the Fallen, including a Resilient Captain. The Guardian continues to search for Saint-14 and finds him in a Ward of Dawn, surrounded by the Fallen.
Saint-14: The Light the Traveler gave to us — it only brings more violence.
The Guardian defeats several Fallen.
Saint-14: Your strength is a sight to behold. But what has all your strength come to? Our people are dying on Earth. They died here, on Zephyr Station.
The Fallen launch signal flares into the air. A skiff deploys a House of Rain Walker and a House of Rain Servitor alongside Fallen reinforcements. The Guardian defeats the House of Rain Servitor.
Saint-14: I arrived with a dozen human colonists: Mercury was supposed to be a garden world. But the Vex were here first. They have murdered this planet. And these Fallen, of the House of Rain, tracked us here all the way from the Cosmodrome. They destroyed us.
The Guardian defeats the House of Rain Walker. Saint-14's Ward of Dawn disappears.
Saint-14: We never should have come. Another minute and they would have eaten my Ghost. Fallen are monsters.
Ghost: Some are. Are you fit to fight, Titan?
Saint-14: I was supposed to protect these people. I should be dead.
Ghost: You're stronger than you think.
Saint-14: Not strong enough.
The Guardian and Ghost give the Perfect Paradox to Saint-14.
Saint-14: What is this?
Ghost: The Perfect Paradox. Built by my Guardian out of spare parts and Light and sheer will to aid you.
Saint-14: It's beautiful!
Ghost shows Saint-14 a hologram of the Last City in the future.
Ghost: I probably shouldn't be showing you this, but when has that ever stopped us? This is the Last Safe City of Humanity. Hundreds of years from now. During the day, there are children laughing in the streets. When night falls, the people sleep in their homes. Not against the walls — weapons in hand — like the early days.
Saint-14: Like my people.
Ghost: These are your people, Saint. Their descendants. If you quit the fight, maybe you'll live forever. Your Ghost will protect you no matter what. But this Last City might never happen.
Saint-14: Everything I've ever built has died. I've buried most of the people I've met. I — I can't do this. Not anymore.
Ghost: We all make our own choices. Good luck, Titan.
The Guardian is teleported back to the Sundial in Mercury's present.
Sundial Slipway, Mercury
Ghost: Okay, let's check the Tower databases to make sure we didn't just wreck the entire timeline. Queuing "Saint-14"… Records state he was a former Commander of the Tower. He vanished on a final mission to Mercury in search of the exiled Warlock Osiris. Well, those are the big beats. Timeline intact. Good job, Saint. But our trip's not over. That broadcast I picked off the Perfect Paradox marked one more set of coordinates within the Sundial.