Mentalis was a war computer. Mentalis was built by Drax at the behest of the Shadow to direct the war between Atrios and Zeos. 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. It was on a countdown, so the Doctor employed a time loop to prevent it. He returned later with Drax, who disabled it completely.