The Skith were thin humanoids made of a crystal that resembled and felt like ice, but wouldn't melt when exposed to heat. Most were transparent, but some were different colours, including the red leader and purple general. They were capable of shooting ice shards from their hands. (COMIC: The First)
Named Skith technology included the Mindcore, a telepathic device which stored the sum of Skith knowledge. They also had space ships and personal transports, (COMIC: The First) and were once able to built a more primitive version of a TARDIS based on the Doctor's knowledge. (COMIC: The Age of Ice)
The Skith were a race of explorers and scientists, attempting to catalogue the life of the universe. However, they had the credo that if you were not the first, you were nothing; if they turned out not to be the first alien beings to discover worlds, they deemed any knowledge gained from such a world to be worthless. They would subsequently use their spaceships to freeze the planet, and convert any sentient life that remained into Skithself, leaving them to be commanded to kill each other or be recovered as specimen samples. This happened to the home planets of the Viskili, the Byndalk and the Mammox.
The Skith ship Oppressor One went on an expedition to Antarctica in 1915, interrupting the Imperial Trans-Antarctic Expedition of Ernest Shackleton. The scientists' experiments converted a whale into Skithself, but the whale escaped, and its morphic field still being flux also temporarily transformed one of Shackleton's expedition, Robert Clark, into a Skith when he came into contact with it. Their discovery of the Tenth Doctor's TARDIS made them conclude that Earth was already exposed to other aliens. Oppressor One began freezing the Earth, but the Doctor, helped by Shackleton, stopped the Skith by sending the ship into the sun. The Mindcore retreated back to the Skith Ownworld. (COMIC: The First)
Sometime after, the Skith's home planet was obliterated by the Crimson Hand. A portion of the Mindcore on a Skith colony survived, and used the ship Oppressor Two to hunt down the Doctor for perfection of time travel to combat the Hand. They returned to Earth and invaded Australia in 2010. There they successfully captured the Doctor and Majenta Pryce, but were defeated by forces of UNIT during a dispute between ship commanders. (COMIC: The Age of Ice)