The Krill were a species created to be a super weapon.
The Krill were humanoid in basic appearance, but much larger than a human. They were covered in armour plating and a layer of spines. Their heads were fish-like, with wide gaping mouths filled with fangs. The Krill had powerful claws which could slice through metal.
The Krill were dangerous because of their considerable resistance to death. Their cells were unspecialised, which meant any cell could do any function and there were no vital organs. They could continue to fight even if they were dismembered, and limbs that were severed would continue to attack. They would continue to regenerate if they were damaged. (PROSE: Storm Harvest)
Krill started life as eggs. These eggs would be launched into space by the inhabitants of Coralee. They could survive all the rigours of space until they landed on a planet. Once there, they would be awakened by radiation, which would cause them to grow into their adult stage. If they took too much damage, they would revert to their egg stage and regrow from there. (PROSE: Storm Harvest)
The Krill were created by the original inhabitants of Coralee. They would send the eggs towards nearby planets, allowing the Krill to kill all the inhabitants. Eventually, some of the Krill eggs were brought to Coralee, and the Krill turned against their creators. They attempted to create a weapon that could destroy the Krill, but they were killed before they could use it. The Krill eventually died out as well, running out of food and things to kill, but leaving behind their eggs.
Many thousands of years later, the Cythosi learned about the Krill and decided to try to use them in their war against the Zithra. They sent a sleeper agent, Philip Garret, to the planet to lead to the development of the human colony. After 20 years, they returned to test out their weapon. They let a radioactive particle awaken a group of Krill, which set out to kill all the colonists. After casualties on both sides, the Krill were killed when Garret was able to use the ancient weapon. The Cythosi found that the Krill were acceptable, and attempted to transmat their eggs to their ship. Unfortunately, Garret sabotaged the nuclear power plant they were doing so in, which awoke the Krill and released them on the Cythosi ship. The Krill killed nearly everyone on-board, though the Seventh Doctor was able to send the ship to the asteroid belt in Coralee's system, saving Coralee.
The Master rescued six Krill eggs from space and brought them with him in his search for the Warp Core. At the opportune moment, the Master unleashed them upon the crew of the starship Gallery, and they later began to battle the Warp Core, which was manifesting on Duchamp 331. The Master planned to have the two forces battle so that he could entrap the Warp Core. Any Krill were presumably killed with the destruction of Duchamp 331 and the Warp Core. (AUDIO: Dust Breeding)