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The Wyrresters were described as being scorpion-like in appearance, though they were much larger than any scorpions found on Earth. The Doctor described them as being "(a) mutant arachnid species" and as being intelligent and dangerous. They had six legs (unlike Earth arachnids, which have eight), and large claws. At least some individuals of the species were intelligent enough to be described as "brilliant." There were at least two body morphs, one being larger and possessing more armour than the other, and perhaps functioning as a soldier class. As a species Wyrresters were intelligent enough and socially organised enough to build large cities, spacecraft, and a teleportation system capable of moving individuals to locations billions of miles away (e.g., Earth). (PROSE:The Crawling Terror)
Wyrresters were native to the planet Typholchaktas, located billions of miles from Earth. When they were encountered trying to invade Earth, their species had overpopulated Typholchaktas and was on the verge of extinction. According to the Doctor, the Wyrresters were already responsible for the extinction of several other species, and the decimation of several planets, during their existence. The first Wyrrester came to Earth when a teleportation device was activated by scientist Jason Clearfield near Ringstone, England, on 21 March 1944. The first Wyrrester was destroyed in a Nazi bombing raid, but it influenced Clearfield's mind and physiology, making him more intelligent and nearly immortal. He spent the next 70 years preparing a new but modified teleportation device to allow the Wyrresters to return to Earth. This was meant to occur on 21 March 2014 but the Wyrrester attempt to take over the Earth at that time was narrowly averted through the efforts of the Doctor, Clara, and several others. (PROSE:The Crawling Terror)