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. Since Leela had communicated with Xoanon, it was suggested that she could help unite the two tribes. Leela refused and asked the Doctor if she could travel with him; when he said no she ran into the TARDIS and pressed a button that began dematerialisation. (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)
When an elder of the Sevateem died, it was customary for other warriors to guard their body in order to protect their soul from evil spirits until sunrise. When Leela's mother was killed in defending Leela in an animal attack, Leela stayed with her body.AUDIO: Empathy Games)