Prothean Beacon

Prothean Beacons are ancient pieces of Prothean technology. They are exceptionally rare, as very few pieces of Prothean 'paleo-technology' have survived intact in the millennia since the Protheans vanished.

Beacons were a system of near-instantaneous communication across the Protheans' galaxy-wide empire. They were made to interface with a person's brain, rather than through audio or video. As a result only Prothean minds can correctly comprehend their messages. An example would be Commander Shepard's 'vision', which was actually a warning about the Reapers sent by the researchers on Ilos, in an attempt to reach any survivors. This explains why Saren Arterius and Shepard need the Cipher, which will enable them to see the world as a Prothean.

There are two beacons in the Mass Effect universe that have been revealed to us: one on Eden Prime and one on Virmire. The message sent through them was coded specifically so that only organic minds could understand it. The Protheans hoped this would prevent any remaining Reapers from accidentally finding out that Ilos had survived. However, at that point, the Protheans did not fully understand the process of indoctrination, and did not realise the message could be used by an organic agent of the machines.

The Eden Prime beacon was badly damaged. Not only did it respond without warning when approached, the 'vision' it held was confused and incomplete. The strain of transmitting it to Saren and then Shepard in a short space of time caused a system overload and the beacon exploded. Liara T'Soni comments that Shepard is exceptionally strong-willed, making her feel exhausted after touching the commander's mind because humans instinctively resist the joining process. Therefore the system overload could have been caused by Shepard instinctively fighting the beacon's vision, instead of welcoming it as Saren did.

The beacon in Saren's lab on Virmire was intact and had been hooked up to a console: Shepard was able to cope more easily when the vision was transmitted. It isn't clear whether Saren found this intact beacon on Virmire or had it brought there from another planet.