Cythosi

The Cythosi were a species of large, bulky humanoids.

Biology
The Cythosi were large, bulky creatures, native to a planet of heavy gravity. They had heavy brows, hide-like skin and bone knobs around their body. Their bones were as tough as titanium and their skin was extremely thick. Cythosi had purple blood. (PDA: Storm Harvest)

Technology
The Cythosi were capable of making a Cythosi into a Human as a form of double agent, though it did cause schizophrenia.

Cythosi would use maintenance bots, small robots with a variety of tools for various situations. They could be programmed to act as defenders or attackers. (PDA: Storm Harvest)

Culture
The Cythosi had a strong sense of pride, and would never back down from a fight. If they were offended by someone, they would personally kill them, even if that wasn't a good idea.

The Cythosi would keep a large amount of slaves for work. If the Cythosi became bored they would torture, kill and eat their slaves. As such, they would rapidly run out of slaves, requiring them to have over 6 times as many slaves as Cythosi in order to compensate for the attrition. (PDA: Storm Harvest)

History
For hundreds of years, the Cythosi were at war with the Zithra. They had been fighting them for generations, constantly trying to stop them but never suceeding. Eventually, they learnt of the Krill and decided to try to use them in their war. They sent a sleeper agent, Skuarte/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 20 years, the Cythosi returned to Coralee. They let a radioactive particle awaken a group of Krill, which set out to kill all the colonists. At the same time, the Cythosi agent became to suffer from schizophrenia from his long mission. He started to go mad, attacking people and eating Edwin Bryce. He also took the only weapon that could kill all the Krill, and eventually used it. The Cythosi found that the Krill were acceptable as weapon and came to the planet to, ostensibly to help repair the damage, but actually to transmat the Krill eggs to their ship. Unfortunately, Garret sabotaged the nuclear plant they were doing so in, which awoke the Krill and released them on the Cythosi ship. The slaves on-board took advantage of this and staged a revolt, leading to major difficulties for both groups. Though the Doctor tried to make peace, the Cythosi refused to abandon ship and were killed, while the slaves had to escape. The Cythosi ship was destroyed to get rid of the Krill. (PDA: Storm Harvest)