Sentient

The Sentients are (or were) a race or force or group that were the Orokin's enemies in the Old War. Infamously little is known about this race, being the most obscure part of the lore of Warframe, but from their name it is presumable that they could feel and perceive things; it is suspected that they were a truly alien race (whereas the Orokin, Grineer, Corpus and Tenno all seem to be human or post-human variations).

In war, they used some kind of heat-based beam weapons and "worm-ships" ringed in glowing discs; these may have used a similar method of propulsion as Orokin ships, since they are referred at one point as "coming out of punch" (an expression denoting the use of a Solar Rail). It is also known that they had heads (which could be torn off with sufficient force) and that they participated (and died) in battle in large numbers. It is unknown where the Sentients came from, how they came into contact with the Orokin or what began the war. At least one battle took place around a "dark and blinding" blue alien star. What is known, however, is that they had the ability to subvert any technology or advanced weapon that was used against them. Since the Orokin were an eminently technological faction, this forced them to use primitive weapons to fight them, "zero-tech" equipment like percussion rifles, "Splinter ships" and bladed melee weapons, which ultimately proved ineffective.

It is not known for certain if the Infested the Orokin engineered were effective against the Sentients, but they were most likely not since the Orokin resorted to their most desperate option: the Tenno. People who originally came into direct contact with the environment of the Void and survived, becoming afflicted in an unspecified manner while also possessing powerful abilities (which not even the Orokin fully understood), they were trained and equipped with Warframes, specially designed armor suits created to contain and channel their power. Thus the Tenno, regarded as monsters, apes, demons, demon apes, ghosts or mere propaganda myths by many people of the Orokin Empire, were made into the perfect warriors to fight the Sentients, since they were not reliant on advanced technologies, but rather on their own Void-derived power.

The final fate of the Sentients is unknown, but the Tenno returned victorious to their Orokin masters in Terminus and apparently slaughtered them all.