Planet E9874 supports a developing civilisation known as the Tarl. The peaceful, technologically advanced Locoyuns are helping the Tarl develop rudimentary technology. What could be more innocent than that?
When The Doctor, Leela and K9 arrive, they find the delicate balance in the relationship between the two cultures reaching an unexpected crisis point. The spears are flying and the threat of all-out war is in the air.
The Doctor must use all his guile to tread a careful path with Tarl leader Ergu, while Leela and K9 discover an ancient power of unimaginable strength which threatens to tear the minds out of its victims.
The Exxilons was the first story in the fourth series of the Fourth Doctor Adventures. It was also the first appearance of the Exxilons in any medium since their debut in 1974.
the Third Doctor previously encountered the Exxilons on their home planet Exxilon. (Death to the Daleks) the Seventh Doctor would later encounter the psychevore Huitzilin, partially created through mutation from an Exxilon device. (The Left-Handed Hummingbird)
Leela refers to the Tesh. The Face of Evil)
The Doctor notes that he saw the markings featured on the beacon in both a temple in Peru and the Great City. (Death to the Daleks)
Official The Exxilons page at bigfinish.com