Geth Prime, aka Geth Terminator, was a Geth in charge of communications and analysis of foreign signals on a Geth starship. He became fascinated with human culture and loved the Terminator series of films, and changed his visage to match Arnie in his Terminator form, even getting an extra set of red eyes. He eventually jumped ship at a spaceport.
He travelled the universe as a mercenary but found it hard to sever his link with the Gethie collective. He eventually returned to Geth-controlled space but was officially outcasted because he was considered a 'renegade'. Outraged, he declared war on the Geth and slew hundreds.
The Geth appealed to the League for help, but League officials claimed the problem was theirs alone (historians call this harsh but it must be remembered that they were dealing with the Reapers at this point and had no time for petty in-fighting).
Eventually the Geth fleet amassed over Raxus Prime, a junk planet on which the Terminator Geth was hiding. They were readying to glass the planet but they stopped when they realised that, despite being a junkworld, Raxus Prime still had a minor population. So instead they sent down strike teams, but all were slain by this incredibly advanced and upgraded Geth Mercenary.
However, at this point the Galaxy began to take notice. News reports from Raxus Prime beamed into every home in the Civil Space and they cried out for action. Many brave Geth were dying at the hands of this monster and the League was sitting and doing nothing.
Eventually, a team of volunteer forces mostly consisting of Elites and Turians but also containing some Humans, a few Geth and even one Slitheen landed on the planet to engage this monster. At this point the Terminator's sanity had failed, and he was ruthlessly slaughtering the innocent population of Raxus Prime. The strike team hunted the Geth down and engaged him in battle in the ruins of a destroyed mining ship, the carcass of which had been rotting on Raxus Prime for thousands of years.
They used an advanced form of weaponry lent to them by the Daleks to disintergrate the ruined ship, but the Terminator Geth survived. His shields in ruins, he charged on the strike team, and killed twenty-one members of the fifty-member company before finally being brought down by rockets launched by three Humans.
The Geth Terminator's body was taken back to Geth-controlled space to be analysed, but no cause of his rampancy was ever found. His condition forever remains a mystery, and the answers remain lost after his destruction. Geth blame his defection on a manufacturing fault but many League members believe he was some kind of new advanced warrior AI that went rogue. Whatever his origins, he will forever be remembered infamously for his reign of terror over the Galaxy, and the Geth see him as not just a criminal but also an example of the dangers of independance and free-will.