The Sevateem were the degenerated remnant of the original expedition's survey team. Their development and behaviour were influenced by Xoanon as part of its experiments to contrast physical and intuitive development with mental and psychic development.
They lived in a concentrated environment, afraid to travel beyond its borders for fear of phantoms that, in actuality, were projections of Xoanon's psyche. Banishment was one form of punishment. The tribe also kept a pit full of Horda used for executions.
The attack was a failure and provoked criticisms of Neeva. The Fourth Doctor used the communicator that allowed Neeva to speak to Xoanon to instruct him to send the Sevateem into the Tesh base. There, the Doctor was able to restore Xoanon's sanity, ending his attempts to control the people and putting an end to the phantoms. The Doctor left the Sevateem and the Tesh to their own means as they attempted to unite their two tribes. (TV: The Face of Evil)
Leela was the greatest warrior in the history of the Sevateem, who sacrificed her life to save her tribe. (AUDIO: The Catalyst) The Doctor's companion Leela was named in her honour. (AUDIO: The Catalyst, Arbitration, The Abandoned)
The Sevateem had a warrior culture, living in a state of permanent war with their counterparts the Tesh. Men and women were both expected to fight. Crossbows, knives, and poisonous janis thorns were all common weapons. The test of the Horda, a trial by ordeal involving vicious native predators, was also part of their way of life.
The tribe's culture revolved around the worship of Xoanon, who they believed was held captive by the Evil One (a reflection of the computer's damaged psyche). The Evil One supposedly resembled the Doctor's fourth incarnation. The tribe's priest had access to Xoanon through an almost-derelict communications array. Many other relics hinting at the Sevateem's technological origins were a part of daily life, including an ultra-beam accelerator, although the tribe themselves were not aware of this. (TV: The Face of Evil)