Tiisin

The Tiisins are a sapient alien species from a planet that once belonged to the Sol System. Made out of "ultraviolet substances", the Tiisins and their cities are both invisible to Humans under normal circumstances.

Biology
Tiisins are bipedal creatures with light-grey hairless skin, and stand about seven feet (~ 2.13 meters) in height. From the neck down they're mostly humanoid, but their heads have a double set of ears, as well as a long proboscis. A pair of large eyes glows with phosphorescence in the dark. By making eye contact, they're capable of communicating with other species by transmitting telepathic thoughts and images.

In life, both the Tiisins and their buildings are normally invisible to Humans, although the reverse is not true. When they die, their bodies suffer a quick physiological degeneration that turns the ultraviolet pigments into infraviolet ones, and makes them become visible upon death.

The lives of the Tiisins are closely linked to an oval stone-like object known as the Doir, which originates from their destroyed homeworld. They feed on the light rays that emanate from the Doir, and are also connected to it in some intricate way. With the Doir intact, they can live for thousands of years, but without it, they die soon.

Culture and society
After their homeworld was destroyed in an undisclosed cataclysm, a group of Tiisins fled to Earth and built an invisible city called Ciis in the Lob-nor desert in central Asia. Although not openly hostile, the Tiisins are distrustful of Humans and prefer to stay hidden.

They have domesticated a blood-sucking animal that looks like an elongated starfish with swollen tentacles and a head equipped with a curving perforating proboscis similar to a mosquito's. Like the Tiisins themselves, this animal is usually invisible, but its body becomes visible in death.

Appearances

 * "The Invisible City", by Clark Ashton Smith (1932)