Mercury and Sand-Dwellers
The Sand-Dwellers, a nocturnal, sentient species, flourish near the night side of Mercury, where their large eyes and ears assist them in hunting and gathering in the near darkness of Mercury's icy night. Sand-Dwellers are capable of hibernating deep underground beneath the burning sands of Mercury's day side.
Sand-Dwellers are essentially apex predators of Mercury's night time surface, but while hibernating underground by day they are vulnerable to being attacked and eaten by mysterious subterranean tentacled Mercurian Cthonians, which Sand-Dwellers have been known to try to appease with sacrifices of their own people.
Based partly on a silicon-based chemistry, their species has made use of a peculiar form of silicon crystal-based "biotechnology" which they have used for many remarkable purposes ranging from generating warmth in the icy darkness of Mercurian night, to opening portals through time and space onto other worlds. Their strange "bio" chemistry renders them vulnerable to sunlight: their occasional journeys by portal into Human-occupied Earth may have led to human legends of strange nocturnal bogeymen and trolls turned into stone by the rising sun - victims of Earth's far milder daylight climate.
Sand-Dwellers play the important role in the Mercurian ecology of carrying seeds of Mercurian plants deep underground with them, where the seeds, protected from the deadly sunlight of this world, can germinate and grow by twilight, blooming as they reach the sun-baked surface, beginning their strange life-cycle anew as night falls and the Sand-Dwellers rise up from the depths to harvest the alien crops.