- "We destroy, we create. That is our function."
The Sou(ou)shi are a spacefaring predatory species which only preys on sapient life, feeding on the mental energy of entire races. Their natural form is about fifteen feet tall, vaguely similar to a praying mantis, with long tusked jaws and four arms terminating in sharp claws; however, they are able to project an illusory humanoid appearance which is far less threatening in order to fool their victims. The Sou(ou)shi function as a collective hive mind, and are psychologically incapable of telling lies. The Doctor has described them as psionic vampires, and wonders if they are related to the Great Vampires, but the Sou(ou)shi appear to have never heard of them.
Their hunting strategy consists on finding inhabited dying worlds, promising to relocate the population and attempting to get their permission to be preyed upon by not revealing the later stages of their plans, which include the slaughter of the prey species and the use of their resulting detritus to fertilize new planets where sapient life could one day arise, proportioning new food for the Sou(ou)shi. Owners of a decidedly alien moral code, the Sou(ou)shi believe there is nothing wrong with this deception, that it is a valid way to seek the prey's permission to be fed upon. It would be wrong for them to feed on a species without its permission, and it would be almost impossible to acquire such permission without concealing the truths that the prey would find unpleasant. For the Sou(ou)shi that is the only way of diplomacy.
So weird is the psyche of the Sou(ou)shi that they genuinely regard the death of other species as a positive thing, a necessary part of the cycle of destruction and creation that they have been practicing for centuries. To them, compassion is considered an evil habit, since it leads to "unnecessary survivals".
About three billion years in the past, their attempts to prey on the Venusian species failed and their true intentions were revealed, after which they were physically destroyed, but somehow survived as a disembodied intelligence and went dormant within the crust of the ancient Earth, waiting for the time sapient life will evolve on the planet and unearth them.