The Zha'til were a synthetic race that existed in Mass Effect.
The zha'til were a synthetic race that existed at 50,000 years ago. They were created by a race known as the zha who implanted the zha'til within themselves to become symbiotic with AI technology. They were made to enhance the zha's abilities in order to survive when their homeworld became inhospitable.
After the Reapers arrived for the Cycle of Harvest, they subjugated the zha'til, corrupting them to seize control of the bodies of their masters turning them into slaves for the Reapers.
The Reapers indirectly caused the Metacon War that erupted between the zha'til and the Prothean Empire to ensure that synthetics would always turn upon their masters. Thus giving a casus belli for their so-called harvests. With no other recourse, the Protheans had to send the star of the zha's home system into supernova, destroying the zha'til entirely.