Master Chief and Cortana managed to escape the terminus through the portal.

Master Chief: What were those things?
Cortana: I'm not sure, they were some sort of advanced defense AIs. Related to the Sentinels, I'm guessing, but it's hard to say without a closer look. C'mon, let's figure out where that transit system dumped us.

Master Chief went outside the room and approaches the console he inserted the chip into the console as Cortana appeared.

Master Chief: Where's Infinity?
Cortana: That's Requiem's core, but Infinity is not here.

Cortana brings up a holographic panel in front of her.

Cortana: That satellite in the center is amplifying the ship's broadcasts like a relay.
Master Chief: Maybe we can use it to respond.
Cortana: Perhaps.

Cortana access several other options on the screen.

Cortana: Those beams coming off of it are creating the interference we've been experiencing. We'd have to take them out to contact Infinity.
Master Chief: Can you get us there?
Cortana presses a final prompt, opening a nearby translocation portal.

Cortana: Opening a gate to the first beam pylon. Pull me, and let's go.

Master Chief retrieves Cortana from the terminal and readies his rifle. As he went into the portal he made it into the hallway.

Master Chief: Is this the first pylon?
Cortana: Negative. This is as close as I could get us. Hope you don't mind hoofing it a little.
Master Chief: Mark it down.

The Chief exits into a tight canyon. A schematic is displayed on his HUD, showing the path to the pylon. As he exits into a small valley, mechanical quadrupeds appeared are crawling over the rocks. The nearby skyline is dominated by the pylon.

Cortana: Contacts!

The automatons hastily retreat as Master Chief approaches. He continues on, encountering more of them in the next valley.

Cortana: And they're back! Coming down the walls!

Master Chief fires with his assault rifle to creatures. When he neutralized the crawlers, he continued his journey.

Master Chief: Those weren't the same things we saw in the Terminus.
Cortana: Similar cortical footprints as the tower AIs... They're connected, all right.

Master Chief activates a hard light bridge. Crossing the bridge, he enters another valley.

Cortana: Look Out!

Master Chief looks up just in time to see a Promethean Knight crashing down from above. The Knight knocks John on the ground, emptying his shields, and screams; its faceplate splits open, revealing a glowing blue skull that is disturbingly human. John punches the Knight's head and then kicks its abdomen, knocking the machine off him. Snarling, the Knight quickly teleports away.

Cortana: Close one.
Master Chief: We better get going.

John arrives at a cliff edge. Knights are seen phasing away from the opposite rocks.

Master Chief: More of them?
Cortana: I guess we're about to get busy!

Then the Firefight began as Master Chief fired with his Rifle and Promethean Knights tried to fight back but they were completely neutralized.

Cortana: I've discovered something interesting about our new friends. When the big ones explode, that momentary flash we're seeing is actually a data purge.
Master Chief: Can you tap into it?
Cortana: They are known as Promethean Knights.

The Chief exits into a large cavern, disposing of more Knights and Crawlers along the way. As he approaches a raised platform, he picks up a garbled transmission from Captain Del Rio over the COM.

Cortana: "The relay interference is increasing. We must be getting close to the pylon."
More Prometheans occupy the area below. After wiping out the opposition, John's HUD is distorted by purple static.

Master Chief: What's that distortion?
Cortana: ...that's me. Something about moving through those portals is increasing the load on my systems.

Master Chief was worried that her rampancy gets much worse.

Master Chief: Are you...going to be alright?
Cortana: Don't worry. I've held off rampancy this long, haven't I?

On the far side of the depression is a rocky path, which leads up to the exit; the second terminal lies below this path. After traversing another tunnel, John finally arrives at the pylon. The walkway leading to the top is blocked by hardlight shields.

Cortana: That's the target, but it looks like the entrance is shielded. Let's figure out how to take it down.

John explores the area, engaging more Prometheans, and eventually finds an energy sphere in one of the smaller structures.

Cortana: One of the shield's power cores. Take it out!

John destroys the sphere.

Cortana: I read two more cores on our level. Hit them before you climb all the way up.
John continues on and destroys the second core in another structure.

Cortana: Well done, Chief! One to go.

John destroys the last core.

Cortana: Great, that's all the cores. Head for the top of the pylon.

John heads up the main structure. Two more Knights phase in to block the pylon entrance, but are quickly defeated.

Cortana: "Chief, look at this:"
A schematic is displayed as another garbled transmission from Del Rio plays in the background, displaying the Infinity approaching Requiem's entrance portal.

Cortana: "That's Requiem! They're not inside at all...they're moving into orbit."
John enters an elevator, which starts up to the top of the pylon.

Captain Andrew Del Rio (Video COM): (garbled) This is Captain Andrew Del Rio, hailing any survivors of the UNSC Forward Unto Dawn; we're approaching your last known location.

The video feed is briefly superimposed by the same orange glyph encountered at the Cartographer.

Master Chief: Did he say Forward Unto Dawn?
Cortana: They must have intercepted our distress beacon!
Master Chief: The beacon was pulled into Requiem with us. If they try to follow it...
Cortana: ...they'll get caught in the gravity well. I'll keep trying to warn them; you just get that beam down!

The elevator stops and the doors open. A rotating mechanism is projecting the energy beam into the distant satellite. A small console stands directly in front of the beam emitter.

Cortana: That's the beam control!

John pulls out a metal shaft inside the console, reorient it to a horizontal position, then slams the cylinder back in. The beam emitter deactivates and retracts into the floor.

Cortana: It's working. The signal from the relay is starting to clear up.
Del Rio (COM): FLEETCOM Actual, we've detected a UNSC beacon coming from somewhere inside the planet-

Master Chief: They haven't hit the gravity well yet.

A portal activates on the far side of the chamber.

Cortana: There's still too much interference to warn them. We've got to disable that other beam before they're pulled in like we were.

John is returned to his starting point. With the first pylon disabled, only half of the energy field surrounding the satellite remains active. As the Chief approaches the console, three CRS-class light cruisers and their Phantom escorts drop out of slipspace inside the core. They rapidly close in on the second pylon.

Master Chief: What are they doing here?
Cortana: They're heading to the second pylon
Master Chief: then We better get going.

They entered another portal as they encountered Covenant, Prometheans as He neutralized them until he made it to the Second Pylon.

Rio (Video COM): UNSC Infinity to survivor, Forward Unto Dawn. Reading a faint IFF tag near the planetary core, do you read?
Cortana: The planet's core? They know we're here! Infinity, this is UNSC AI Cortana. Do not approach Forerunner planet! Repeat, do not approach-
Del Rio (Video COM): Infinity to UNSC asset! Forward Unto Dawn! We read you, but you're breaking up! Helm, increase speed by point-seven. Get us in there.
Cortana: Negative, Infinity! Do not enter the planet!
Del Rio (Video COM): If you can hear us, keep transmitting.
Cortana: No! Chief, you've got to get that beam down now!

The elevator stops, and John emerges into the emitter chamber.

John deactivates the second pylon, using the same method as the first.

Cortana: Cortana to Infinity, do you copy? Come in, Infinity! The interference is gone, but your suit's transmitter's not strong enough.
Master Chief: Route us up to the relay satellite.
Cortana: Already done! Go!

He made to the satellite controls and neutralized the Covenant and Prometheans.

Cortana: Wait! Something's not right-
Master Chief: We don't have time!
Cortana: The pillars! Touch the pillars!

Master Chief places his hands on each pillar.

Master Chief: Infinity! This is Sierra-117 of the UNSC Forward Unto Dawn. Do you copy?

John's own speech is distorted and amplified back at him. The sphere slowly rises, emanating an orange glow. John stares at it in confusion.

Master Chief: Infinity? Do you read?

Again, John's words are distorted and amplified back to him. The sphere continues to rise, its black plating folding away to reveal a glowing orange core. Below the Chief, both Covenant and Promethean forces cease fighting and look upwards as the Cryptum is set free.

Cortana: Chief, Let go of the contacts!

John quickly pulls his hands away from the pillars, proceeding to ready his assault rifle.

Master Chief: Find us an exit.
Cortana: Don't wait around on my account...

John leaps off the platform just as the Cryptum releases an orange energy wave, knocking the Covenant and Prometheans off their feet. John takes cover behind a small barrier, peeking over to observe the sphere.

Master Chief (thoughts): I have a bad feeling about this

A small triangular capsule lowers itself out of the Cryptum's bottom opening. The capsule opens up to reveal a kneeling humanoid figure. Upon seeing him, John ducks back behind the barrier. The figure stands up and flexes his muscles. His customized combat skin assembles itself around him. Once fully armored, the figure assesses his surroundings.

Didact: So fades the great harvest of my betrayal.

The Ur-Didact waves his hand, creating a shimmering wave over the nearby Prometheans; their blue highlights turn orange, signifying his control over them. The Sangheili bow down in reverence of their living god.

Jul 'Mdama: Didact!

The Ur-Didact's platform floats toward the Chief's position. He looks at John disdainfully.

Didact: Even these beasts recognized what you were oblivious to, human. Your nobility has blinded you...as ever.

The metal barrier shielding John disintegrates. John immediately raises his rifle, but the Forerunner simply reaches out and telekinetically binds him. The helpless John is lifted to face the Didact.

The Librarian left little to chance, didn't she? Turning my own guardians - my own world - against me. (looks down at the Prometheans, then looks back at John) But what hubris to believe she could protect her pets from me forever. If you haven't mastered even these primitives (looks down at the Sangheili), then Man has not attained the Mantle. Your ascendance may yet be prevented.

The Ur-Didact pulls John in closer.

Ur-Didact: Time was your ally, human. But now it has abandoned you. The Forerunners... have returned.

The Ur-Didact summons his helmet, which assembles itself to resemble a glowing metal skull. John struggles futilely against his grip.

Ur-Didact: This tomb...is now yours.
The Ur-Didact tosses John away, slamming him against a wall. He crashes onto the floor. The Spartan groans and forces himself back up, seeing the Ur-Didact retreat back into the rising Cryptum, now surrounded by swirling energy.

Cortana: Slipspace rupture! Chief, move!

John runs for his life but it was too late The Cryptum's energy glows increasingly bright, then explodes with a blinding flash.

Cortana: Chief, please! We've got to go!

Fade in to John's HUD, slowly coming to. He's back in the canyon.

Cortana: That...Didact. He manipulated Infinity's signal to get us to release him! Get up!

John regains full consciousness and quickly forces himself up.

Master Chief: What's happening?

John looks up to see a swirling energy field forming above him. A light cruiser is sucked into the breach, alongside masses of debris.

Cortana: Moving the satellite into slipspace destabilized the core.
A nearby Phantom is pulled into the field as well, smashing into another Phantom in the process. Both crash into the canyon walls, their Ghost complement spilling out ahead of the Chief.

Cortana: The Didact's leaving! We've got to find a portal out of here before the whole network collapses!

Master Chief quickly took an intact ghost and start driving around the canyon until he entered the portal.

A portal opens near a rocky cliff, spitting out John's Ghost. The edge of the cliff is rapidly approaching.

Cortana: Whoa, cowboy!

Master Chief quickly vaults on top of the Ghost, then kick-jumps off of it, coming to a stop inches from the edge. The Ghost plummets into the fog below and explodes.

Cortana: Chief, there!

John looks up and sees the massive UNSC Infinity emerging from the clouds, electricity sparkling around the hull.

Master Chief (thoughts): We're too late.

The ship leaves the cloud layer and draws closer to the Chief.

Del Rio (COM): Mayday! Mayday! This is the captain of the UNSC Infinity. Unknown entity has seized control of our ship! We're without power, and on a collision course with an unidentified Forerunner planet!

The Infinity speeds past the Chief's position and descends into the jungles ahead.

Master Chief: Track its descent.
Cortana: Marking. Impact predicted 77.8 kilometers due north.

John watches as the Infinity descends. The air begins to waver and distort behind him as the Cryptum rises from below the cliff edge. John quickly turns back and readies his assault rifle in alarm. The Cryptum sends out a scanning pulse, then zooms away to the Infinity at supersonic speed.

Cortana: You know where he's heading...
Master Chief: Same place we are

He starts walking toward the same place where the wreckage of the Dawn at.