Ilos

Ilos is the planet used to house a Prothean research facility. It can be accessed via the Mu Relay, and is visited during the final parts of the game.

The Protheans who were in the research facility were aware of the Reapers. They built the Conduit on Ilos in an attempt to break the cycle of extinction. However, they failed, and attempted to send out a warning via the beacons Commander Shepard's vision was the warning the researchers tried to send out. The player would see the vision more clearly after the mission in Feros, where they would gain the cipher.

When the Reapers attacked, Ilos was somehow spared.

What was left of it in Mass Effect
It is now a ruin. However, it somehow still has power to power its elevators and Vigil. In its walls are the bodies of the researchers. They attempted to cryogenetically freeze themselves until the Reapers have gone, but the machines stayed in the galaxy for centuries, and in the end, Vigil shut down every pod until only the top scientists were left.

Plot in Ilos
Commander Shepard enter Ilos with some difficulty, as his ship was grounded due to the Coucil fearing that he would go to Ilos (If Commander Shepard was discovered there, it would mean an all out war between the Terminus Systems and the Citadel). However, he still managed to go there as Captain Anderson overrided Ambassador Udina's order. Commander Shepard lands on the planet despite complications, but was not in time to go through a large door that leads to a room Saren retreated into just in time. He then find Vigil after some run ins with the Geth, and everything was then explained. Commander Shepard then found the Conduit in the nick of time. To add to the tension, cutscenes reveal that Saren has already arrived at the Citadel started his assault. The time in Ilos ends when Commander Shepard made a mad rush towards the Conduit, effectively transporting himself into the Citadel through the Mass Relay 'statue'.