The Cythosi were a species of large, bulky humanoids.
The Cythosi were large, bulky creatures, native to a planet with high gravity. They had heavy brows, hide-like skin and bone knobs around their body. Their bones were as tough as titanium, their skin was extremely thick and their blood was purple. (PROSE: Storm Harvest)
The Cythosi could turn a Cythosi into a human as a double agent, although this caused schizophrenia.
The Cythosi were proud, and would never back down from a fight. If they were offended by someone, they would personally kill them, even if it was not a good idea.
The Cythosi kept many slaves. If the Cythosi became bored, they would torture, kill and eat their slaves. As a result, they rapidly ran out of slaves, requiring them to have over six times as many slaves as Cythosi to compensate for the attrition. (PROSE: Storm Harvest)
For hundreds of years, the Cythosi were at war with the Zithra. Eventually, they learned of the Krill, and decided to use them in their war. They sent a sleeper agent, Philip Garret, to the planet to lead to the development of the human colony. He set it up so that all the necessary events for the awakening of the Krill were there.
After twenty years, the Cythosi returned to Coralee. They let a radioactive particle awaken a group of Krill, which set out to kill all of the colonists. At this time the Cythosi agent suffered from schizophrenia after his long mission. He began to go mad, attacking people and eating Edwin Bryce. He also took the only weapon that could kill all the Krill, and used it. The Cythosi found that the Krill were acceptable as weapons and came to the planet, ostensibly to help repair the damage, but actually to transmat the Krill eggs to their ship. Unfortunately, Garret sabotaged the nuclear power plant in which they were doing so. This awoke the Krill and released them on the Cythosi ship. The slaves onboard staged a revolt, leading to major difficulties for both groups. Though the Seventh Doctor tried to make peace, the Cythosi refused to abandon ship and were killed, while the slaves were able to escape. The Cythosi ship was destroyed to get rid of the Krill. (PROSE: Storm Harvest)