Ilos

Description
In the golden age of the Protheans, Ilos was a verdant world, dotted with the spires and arches of magnificent cities. Even casual observation shows this is no longer the case. Ilos has been devastated by means unknown, its entire surface changed to the color of rust. The atmosphere shows heightened levels of oxygen. Wildfires, presumably ignited by lightning strikes, can be seen burning on the dark side. This indicates that most – if not all – respiring animal life forms have died off.

Codex Entry
Primary → Planets and Locations → Planets: Ilos

Like the ancient human city of Troy, Ilos is a world known only through second-hand sources. References to Ilos have been found at several other Prothean ruins, though direct study of the world is unlikely to occur.

Ilos lies in a remote area of the Terminus Systems only accessible by the legendary Mu Relay. Four thousand years ago, the Mu Relay was knocked out of position by a supernova and lost. Since then, Ilos and its cluster have been inaccessible.

Occasionally, a university will organize an expedition to chart a route to Ilos using conventional FTL drive. These never get beyond the planning stages due to the distance and danger. The journey could take years or decades, passing through the hostile Terminus Systems and dozens of unexplored systems.

Missions

 * Race Against Time → Race Against Time: Sovereign
 * Ilos: Find the Conduit

Plot
"There are secrets here that were meant to be forgotten." - Garrus Vakarian

Rough Landing
Tracking Saren Arterius during his search for the Conduit, the Normandy heads through the Mu Relay to Ilos, but the Council's attempt to ground them means Shepard has lost too much time. A geth fleet is in orbit around the planet. The stealth system keeps the Normandy off their scanners but Saren is already on the surface. Navigator Pressly picks up some strange readings that must be Saren, but there's no landing zone nearby. They can't drop the Mako - they need 100 meters of open terrain and the best Pressly can find is 20 metres, a suicide run. The descent angle is too steep. Joker claims he can pull it off. Shepard, trusting his skills, tells the squad to suit up, and orders Joker to "drop us right on top of that bastard."

On the surface, Saren is leading a troop of geth through a ruined city into a bunker. He looks up to see the Normandy bearing down on him. As Joker drops the Mako in a perfect maneuver, Saren makes it inside and a heavy door closes behind him. The team realize they have to find the security station that Saren used to get past it.

Shepard leads them through the ruins, facing the heavy ground forces that Saren left behind, including several Geth Armatures and Geth Hoppers lying in ambush. They discover the security station and manage to open the bunker door, but as they turn to leave, a damaged VI is activated and starts replaying a garbled message in the Prothean language. Thanks to the Cipher, Shepard can understand it, but the message is too degraded and only a few words are clear: 'Citadel', 'in the archives', 'all is lost'.

Vigil
The bunker door is open by the time they get back to the Mako, revealing the entrance to an underground complex. Driving down into it ("Who votes we take the vehicle into the creepy underground bunker?") the squad see hundreds and hundreds of containers in the walls, inactive cryogenic pods with the inhabitants long dead. Further down, the way is blocked by a forcefield; as the Mako gets closer, another forcefield descends from behind, trapping them. Shepard sees an elevator close by and, hoping there is something friendly controlling those forcefields, takes the elevator down deeper into the archives. In a chamber at the bottom, the squad discover a Prothean VI named Vigil. He is relieved that none of them are machines or indoctrinated slaves. He explains why he has brought them here - Shepard must finish the work the Protheans began and break the cycle of extinction.

Vigil goes on. The Citadel is the heart of galactic civilization and government, but it is also a trap. The station is a mass relay that leads to dark space where the Reapers reside. When the Reaper vanguard, Sovereign, sends a signal to the keepers, the relay is opened and the Reapers return. Using the element of surprise, the Reapers kill the galaxy's leaders on the Citadel and destroy the fleets there before anyone has a chance to stop them, throwing the galaxy into chaos and beginning the cycle of extinction once more. But as Vigil explains, this time there is a chance.

In Prothean times Ilos was a secret facility built to research mass relay technology. The researchers managed to create a small working mass relay that linked to the Citadel, the Conduit. Soon afterward the Reapers invaded through the Citadel. All records of Ilos were lost in the initial attack, so the planet was spared the Reapers' genocide. But the researchers and support staff went into cryogenic stasis in the archives as a precaution, while Vigil monitored them and waited for the Reapers to depart.

But over the centuries while the Reapers systematically destroyed the Protheans, Ilos' energy reserves began running low. One by one the cryo pods were deactivated to save power, until only the top scientists were left. When Vigil revived them, the scientists sent a message through the Prothean Beacons trying to find survivors - the vision both Shepard and Saren saw - and, realizing their species was doomed, tried instead to strike back against the Reapers. The Prothean scientists found a way to change the keepers' physiology so they did not respond to the Reapers' signal, then traveled through the Conduit to put their plan into action.

Now the keepers have evolved so they only respond to commands from the Citadel itself and not any external source. Sovereign tried sending the signal, but nothing happened, and it was forced to change its plans. Sovereign could not attack the Citadel directly without help; revealing itself would have united the organic races against it and Sovereign would have been destroyed. But after centuries of careful planning, it now has Saren to help it. He can travel through the Conduit to the Citadel and gain control from the inside, before handing the Citadel over to Sovereign so the relay can be manually opened.

Aqueduct Run and the Conduit
Vigil gives Shepard a data file that will corrupt the Citadel's security protocols and give the commander temporary control of the station. Shepard must follow Saren through the Conduit, to find the Citadel's master control panel, and upload the file there. It is now their only chance to buy the Citadel Fleet enough time, and to stop Sovereign opening the relay. Saren is on his way to the Conduit: Vigil tells the squad there is still a chance if they hurry.

Going back to the Mako, the squad drive out of the archives and down an old aqueduct, fighting off the geth left behind to slow them down. Finally they reach the Conduit, a miniature mass relay pointing straight up. It is active; Saren has just gone through and left more Armatures and Geth Colossi on guard.

Meanwhile, Sovereign arrives through the Serpent Nebula's mass relay and approaches the Citadel with a huge fleet of dropships. Saren is already on the Citadel, leading his geth troops through the Presidium. Aboard the Destiny Ascension, as the attack begins, the asari crew realize the Citadel isn't responding and begin to evacuate the Council.

On Ilos, Shepard sees the Conduit is beginning to close. Under heavy fire, the commander drives the Mako past the geth and straight into the Conduit itself with only a few seconds to spare. The mass relay fires the Mako straight into the air, back toward the Citadel for the final battle.

Trivia

 * In Greek mythology, Ilos is the founder of the city of Ilion or Illium, which is also known as Troy.


 * The last section of Ilos, where the Mako rushes to reach the conduit in time, is named "Trench run" on the map screen, possibly in reference to the trench run on the Death Star at the climax of Star Wars.