Sou(ou)shi

The  were a monopsiopsychosemiotic species. They could create an illusory appearance, appearing as human, but were 15 foot tall with four arms in their true form.

The Sou(ou)shi fed on mental energy. Their strategy was to find planets whose inhabitants had no space travel and were doomed, usually due to environmental factors. They would claim that they would evacuate the inhabitants to another planet. In fact, they would kill everyone and excrete their genetic material on a new planet, where new life based on the dead race would evolve. The Sou(ou)shi were incapable of lying, and had to receive permission from a member of the race on which they wished to feed. They became adept at not revealing the entire truth of their plans.

The Sou(ou)shi who came to Venus were destroyed when the Venusians discovered their real intentions. They survived as a bodiless psionic force, and drifted to Earth, where they lay dormant waiting for something on which to feed. (PROSE: Venusian Lullaby)

Behind the scenes
In the reference work I, Who, Lars Pearson speculates that the discorporated Sou(ou)shi become the Sentience.