Mentalis was a war computer.
Mentalis was built by Drax at the behest of the Shadow to direct the war between Atrios and Zeos. According to the Doctor, Mentalis was responsible for attack, defence and surveillance of the war, and he called it highly efficient despite being passionless. It was discovered by the Fourth Doctor while on Zeos with Shapp, Merak, Romana I and K9. K9 communicated with it and considered it an interesting conversation. It refused to answer any inquiries about Astra, nor would it allow them access to its databanks. With the war over, it decided it would destroy everything and everyone on both sides.
Mentalis was fitted with a self-destruct mechanism if it felt threatened. This was activated when it sensed the Marshal of Atrios homing in to attack. The Doctor tried to disable it. It became confused and, to prevent this self-destruct, used its own defence weapons against its command centre to destroy itself.
It was on a countdown, so the Doctor employed a time loop to prevent it. He returned later with Drax, who disabled it completely.
Mentalis was extremely powerful and nearly indestructible, capable of defending itself thanks to an automatic defence mechanism as well as producing and directing war material. It was programmed to think of itself as indestructible. (TV: The Armageddon Factor)