The Racnoss appear like massive, red skinned arthropod-like creatures with six pairs of jointed limbs, four to five of which are used for locomotion. Their upper bodies resemble that of a humanoid, making them the vague arachnoid equivalent of a mystical centaur, or Drider. They have a crested skull and three pairs of eyes, and their mouth is human-looking rather than insectile, but contains razor-sharp white teeth. Their abdomen produces a substance similar to spider silk, which can be used to construct vast webs and immobilize prey.
The species travel around in star-shaped vessels with hulls made out of their own silk. Racnoss technology used to make heavy use of Huon energy. After their defeat, the Time Lords largely eliminated all sources of toxic Huon particles in the galaxy, partially to prevent the Racnoss Empire from ever rising again. The last members of the species concealed themselves dormant in a ship hidden at the center of a newly formed planet, which turned out to become Earth. Only the Empress Racnoss remained.
In 2007, a secret drilling operation by the Torchwood Institute accidentally reactivated the Racnoss hidden lair, sending a signal to the Empress to return, so they can rise again and destroy all life on the Earth. The Empress began to rebuild Huon particles in liquid form using the river from the Thames, which where then injested into a living being in order for the particles to catalyse. After this, the Huon energy would be used to awaken the dormant Racnoss babies in a ship at the centre of the Earth.
The Racnoss were defeated by the Tenth Doctor, who released the water of the Thames into the Racnoss base to drown the babies after the Empress refused his offer of peace. The Empress - now the last of her kind and furious at the murder of her children - intended to 'scorch' the Earth. However, she had used up all her Huon Energy and thus, she was defenceless as the British Armed Forces attacked her - destroying her ship and killing the last of the Racnoss.