Prothean

Protheans are an extinct alien species in the Mass Effect Universe. Protheans mysteriously vanished over 50,000 years before Mass Effect takes place, which would place their demise at around 48,000 B.C.

Physiology

The beacon flashback and statues on Ilos show that the Protheans were humanoid in appearance, but with extremely long tentacle-like fingers, toes and "beards".

Your first contact with the race is through the above mentioned beacon on Eden Prime. After that, you learn that the Protheans were wiped out by a strange species of sentient machines called the Reapers, a species believed to be a myth by the Council.

However, you learn through visions and supporting evidence that the Reapers are, in fact, real, and that you have even been in contact with one of them, Sovereign, without even realizing it. When the Protheans reached the apex of their society, they were struck down by the Reapers, continuing a cycle of rising and falling societies perpetuated by the Reapers.

Every 50,000 years the Citadel sends out a call to all Reapers in dark space and turns itself into an ending point for the Reapers to jump back to and start their extermination.

The Protheans created a secret base on the planet Ilos during the Reaper invasion. The facilities main purpose was to hold the most important Protheans in cryo-sleep until the Reaper invasion had finished and then find a way to prevent the cycle. Only twelve of the highest Protheans survived after the Reaper invasion had finished. The Protheans did not expect the invasion to last for as long as it did and as time went by more and more pods containing Protheans had to be shut down due to lack of power. The facility was maintained by an AI program named Vigil. Vigil was programed to wake the sleeping Protheans after the Reapers had gone back into dark space.

The remaining Protheans we able to create the Conduit for direct access to the Citadel from Ilos, and were able to stop the Citadel from sending the recall message out to the Reapers.

Even though they are credited to it throughout the game, the Protheans did not invent the Mass Relays or the Citadel, rather they were created by the Reapers to set up other species in preperation for their demise; the Protheans merely found them, much like the modern races in Mass Effect did.

It has been speculated that the Protheans may have had some influence on Earth throughout human history, particularly ancient Egypt. Evidence to support this include the Pyramid containment units found scattered on many planets that bear an unmistakable likeness to the ones found in Giza. The Mass Relays also bear some resemblance to the Egyptian symbol for the Eye of Ra.

Commander Shepard encounters an unusual Prothean ruin on Eletania. Once activated, it reveals a niche where Shepard can place the 'trinket' which was a gift from Sha'ira. If Shepard does so, the commander is knocked unconscious in a flash of light, and has a vision of the Protheans using an implant to monitor a Cro-Magnon human. The data was apparently stored in the ruin. After the vision, the worried squad members help Shepard up, but the vision was harmless and the commander is fine. Reliving the primitive's memories, Shepard wonders how long the Protheans were studying humans from their base on Mars, and what they learned, before moving on.