Psions are inherently not part of the natural order. Their abilities are anachronistic in a reality that should be explainable and connected to the physics of the world as a whole.
Society: Less than one in a trillion conscious creatures are naturally occurring Psions. With Psions being on the edge of nonexistent, not many realize they do exist none the less.
Few Psions discover others of their kind unless they survive for millennia. Even when they do, those they find have little more understanding of their joint nature.
Many are discovered by those who wish to understand or harness their impossible powers. No such attempt results in understanding.
The closest that can be achieved is replication, and only if a Psion develops the ability to actively gift their power to others or return those destroyed from Beyond.
In the rare society where Psions are known and common, there is almost always a single true Psion who has gifted power to a vast number of others, using the Cosmic Spark Feature to create and control artificial Psions.
Such societies often baffle Novus colonizers, seeming to wield unknown powers that defy all natural laws. Even stranger are the cases where no patron Psion is present on the planet, but artificial Psions still reach out to something and are answered.
These groups, through happenstance more than intention, attract the attention of the Cosmic. Psions possessing abilities on a deific scale, the Cosmic are capable of reaching across the billions of light years to touch the ephemeral creatures that somehow attracted their attention.
The inevitable fate of a Psion who loses the ability to die, the Cosmic are ancient beyond comprehension. Floating in the intergalactic void or at the hearts of galaxies, the Cosmic are more celestial phenomena than living creatures.
Even those who discover their existence can only speculate whether the Cosmic have a society or are even sapient in any way a Novus could understand. When a Cosmic gifts psionic abilities, the reasons attributed to the action are more the projection of the recipient’s beliefs than any recognizable plan from the Cosmic.
Ecology: The abilities of Psions are not part of the physical laws of reality. Scientific study cannot explain their existence, because they are not part of reality.
They exist because reality cannot perfectly maintain the border between itself and what is beyond. At times, against overwhelming odds, the border fails, and a creature touches the Beyond.
Psions are anomalies half way outside reality. They are not true creatures of the Beyond, but are partially aware of themselves within it and can leverage that awareness.
Although a naturally occurring Psion is vanishingly rare, it is possible for one Psion to create another under certain circumstances. The most common method is to gift powers to others via Cosmic Spark, but such artificial Psions do not have the ability to touch the Beyond themselves.
Alternatively, a Psion may develop the ability to revise reality and make it so a creature that is dead is alive again. More than simply restoring a body to life, a Psion capable of this directly forces the nonexistent creature into existing again.
A creature that experiences this is no longer limited to reality, instead becoming caught both as something real and something not real. It becomes a Psion.
Appearance: Any creature that is self aware can be a Psion, so they have no common appearance.
Game Play: Psions are the exception to the reign of science and logic within a true sci-fi setting. They give a way for a character to gain the supernatural Features only explainable within a fantasy setting.
Because of the rarity of Psions, their anomalous nature has little effect on the setting. This has the advantage of making Psions especially unique and inexplicable existences within a setting otherwise logical and scientific.
Alternatively, the game may take place in or near one or more societies familiar with psionics. A civilization with a handful of Psions possessing the Cosmic Spark Feature could use psionics as part of everyday life.
Similarly, a Psion capable of returning the dead to life can create Psions on mass. If some of those Psions possess the same ability, Psions could become commonplace.
Locations like this cease to be true science fiction settings, becoming a hybrid of science fiction and fantasy. Such hybrids can be a lot of fun, but generate a different tone for the game.