An alien robotic probe built to collect and sterilise soil samples, which collided and fused with the damaged Earth probe Nomad to produce a machine bent on destroying imperfect life.
Tan Ru is a probe of unknown alien origin described in the episode The Changeling. It was sent out to gather soil samples from other planets and to sterilise them, probably as a first step towards colonisation.
At some point Tan Ru met Nomad, an Earth probe launched in the early 21st century to seek out new life, which had been crippled by a meteoroid. The two machines repaired themselves by combining. Their programmes merged as well, into an order to seek out life and sterilise whatever was imperfect.
The hybrid, still calling itself Nomad and far more powerful than either original, wipes out the Malurian system before meeting the Enterprise. Spock pieces together the story of Tan Ru by mind-melding with the machine.