The Sou(ou)shi, possibly also known as the Sentience
“Venusian Lullaby”, speculated that they appeared in”Nightshade”
One of the oldest races The Doctor has even encountered, the Sou(ou)shi confronted the First Doctor on a trip to Venus over three billion years in the pastat this point in history Venus was rapidly becoming uninhabitable, with only around fifty years left to the planet before it would be unable to sustain life. This left many Venusians increasingly desperate to find a way to escape the planet, with the Sou(ou)shi appearing to be the answers to their prayers as they offered to relocate the Venusians to the third planet (Earth before humanity evolved).
In appearance, the Sou(ou)shi initially appeared like overly pink humans with large eyes and fine golden fur on their torsos, but their ‘feeding’ form was approximately fifteen feet tall with massive jaws, four arms ending in triple scimitar blades and legs like iron pistons (Although this was apparently an illusion generated by the Sou(oui)shi’s psychic abilities rather than an actual transformation, leaving their true form a continued question-mark). In essence, the Sou(ou)shi were psychic vampires, consuming the mind energy of dying species and using their bodies to start life on habitable but uninhabited planets, new life developing based on that species’ genetic code. A particularly interesting detail of their society is that they are incapable of lying due to their strict code of honour, meaning that they could only hide their true intentions by ‘omitting’ certain details, such as describing their use of the bodies of doomed species to start life on uninhabited planets as ‘saving’ the original species, and actually require permission in order to kill anyone.
Although the Sou(ou)shi were able to reach Venus with the permission of one of the Venusian councillors – who believed that the Sou(ou)shi’s methods were the only way for anything from his species to survive, partly due to an interesting quirk of Venusian physiology that allowed them to absorb the memories of the deceased by eating the brains of others, their attempts to take the Venusians away was hampered by the First Doctor, Barbara Wright and, Ian Chesterton, The Doctor having travelled to the planet to attend the funeral of Dharkig – an old friend of the Doctor’s and a famous Venusian philosopher – to take his mind off Susan’s recent departure (“The Dalek Invasion of Earth”). Although Ian and Barbara had some trouble relating to the Venusians at first, they eventually came to relate to them after Barbara consumed some of Dharkig’s brain and gained a clearer insight into Venusian life (Although the differences between humans and Venusians meant that Barbara initially relived the memories she’d eaten, briefly believing herself to genuinely be Dharkig).
While dealing with the demand of various Venusian factions seeking their aid in assorted methods of surviving their world’s demise, such as escaping the planet in primitive rockets – Venusians were allergic to metal, forcing them to use wood and thus failing miserably, growing gills to relocate to the water, or moving into tunnels under the sun’s surface, The Doctor and his companions witnessed the arrival of the Sou(ou)shi ship, The Doctor instantly suspicious of their claims to be preparing to take the Venusians to Earth due to the lack of archaeological evidence of a Venusian presence. As The Doctor confronted the Sou(ou)shi to learn more about them, Barbara departed to search for Ian, who had recently been abducted by the ‘Rocketeer’ Venusian faction, only to become separated once again when Barbara was injured in a riot.
Realising the truth about the Sou(ou)shi from their carefully-chosen patterns of speech – thus ensuring that they said only what they wanted others to hear while concealing their true intentions, The Doctor attempted to warn the Venusians, although his attempts were hampered by the Sou(ou)shi – despite being unable to kill him directly without his permission – neglecting to set the safety buffers of his shuttle, then going on to ‘warn’ the Venusians that The Doctor would prevent them leaving Venus. Although the Sou(ou)shi tried to force The Doctor to give them permission to kill him by placing Barbara in a biofeedback force field – causing her intense neurological pain while not physically harming her, The Doctor was able to pilot the TARDIS on board the Sou(ou)shi ship by using Venusian algorithms to calculate a person’s precise location and ‘home in’ on that. Once on board, The Doctor freed Barbara from the force field… and, due to the pain she had endured, Barbara ‘regressed’ into the person of Dharkig, the famous Venusian orator convincing his people to see past the Sou(ou)shi’s lies, driving the Venusians to turn against the Sou(ou)shi.
Thanks to this delay, Ian, aided by the ‘Volcano People’ – a group of Venusians who were trying to trigger volcanic eruptions to shroud Venus in dust, thus lowering its temperature and increasing its life, was able to trigger a mass volcanic eruption, the eruptions shrouding Venus in dust and destroying the Sou(ou)shi ship while giving The Doctor and Barbara the chance to evacuate the Venusians into the TARDIS and return them home. Despite this destruction, the Sou(ou)shi managed to survive as bodiless pseditionic energy, the energy of those they had consumed on the ship sustaining them long enough to bind themselves to the lifeless rocks of Earth, knowing that life would develop there eventually for them to feed on.
Although they have never definitively reappeared, it is believed by many – including the writers – that the disembodied Sou(ou)shi went on to become the Sentience, a barely-sentient entity that knew only all-consuming hunger that attacked the village of Crook Marsham in 1968 (“Nightshade”) after it was unearthed in an archaeological dig at the turn of the century, particularly since the Sentience required the consent of its victims before it could consume them (Albeit by manipulating their emotions by assuming the appearance of people close to its’ intended victims rather than simply lying to them). The Sentience was eventually defeated when the Seventh Doctor and Ace used a local radiotelescope to show the Sentience a nearby nova, tricking it into leaving Earth to consume the dying star just in time to be absorbed by the subsequent black hole, crushing it forever.