The Conduit is a Mass Relay located on Ilos that links directly into the Citadel, bypassing all of it's defenses. The Conduit is a small-scale Mass-Relay, as opposed to the larger ones in space that can only transport starships, the Conduit is capable of transporting something as small as a Mako Rover. The Conduit links to the Mass Relay Momument in the Citadel Presidium. Although everyone simply believes it is a sculpture, the Mass Relay Momument is actually a fully operational Mass Relay, and is used by Saren to launch a surprise attack on the Presidium control center.
Through the game Mass Effect, Commander Shepard and the Council believe the Conduit to be a weapon in itself, and are completely unaware of it's location. When Saren Arterius is followed to Ilos, Commander Shepard discovers that the Conduit wasn't a weapon, but a Prothean built Mass Relay used for escape from the Reapers. Saren uses the Conduit to invade the Citadel with an army of Geth and activate the Mass Relay that links the Citadel to dark space, where the Reapers live. Shepard also learns that the true purpose of the Keepers is to activate the Mass Relay, allowing the Reapers to pass through. The Conduit isn't destroyed during the conflict.