Encroaching Darkness

''Well, I'm f***ing stumped. How does one punch a shadow?'' - Monolith

Encroaching Darkness is the term for a species of light-based creatures that exist only in the forms of shadows. these creatures have no physical body from which they exist, and do not have any form of mass. They exist where there is light, but cannot interact with anything while light is present in a sufficient amount.

Description
These creatures can be vaguely seen on any surface which is casting a shadow. Their appearance is vaguely humanoid, but not always, and their movement implies they can phase through walls and objects. There is no way to actively escape them, however as long as one is in the presence of light, one cannot be taken. That said, Encroaching Darkness has ways to remove light.

Encroaching Darkness have a physics-altering effect on the world around them, and this ultimately leads to the death of any lights around. Electricity will begin to actively diminish, and the chemical reaction of flames burning will slowly weaken. Encroaching Darkness does not have to make physical contact with an object to do so, and may even park itself thousands of miles from a star and slowly siphon its burning nature, dimming the light and weakening it. While the destruction of a star often happens well after the destruction of a planet, this act causes daytime to shorten, and light from the moon to slowly vanish. With weakening electrical power and weakening flames, eventually anyone surviving will be caught.

Survivors can extend their time by keeping flashlights on parts of their body, carrying glowsticks, and using gas-powered generators, as the close proximity to what they power ultimately helps in sustaining their current. It is best to keep at least two light sources active at any given time, and light must be touching exposed skin, or else the light will not help.

Once a person is covered in darkness, they vanish instantly without fanfare. It is as if one ceases existing, leaving everything behind. Survivors who managed to return to the other side explain that the world one ends up in is an expanse of nothing. One cannot hear themselves, see anything, or feel the presence of other humans. As all of a persons belongings, including clothing, are left behind when they suddenly vanish after touching the darkness, taken individuals find themselves naked in the dark, cold and without protection. It is noted that the floor feels smooth and cold, which is noticeable on bare feet and makes laying down to sleep somewhat uncomfortable. There is no air current, just stagnant air, so no breeze is ever felt. This situation continues indefinitely, and it is likely that anyone trapped in this world simply stays there until death. Given any living thing is transported to this dimension cannot find any resources, any being regardless of age or gender is left helpless in this world until they die of dehydration or serious injury.

Those who did come back do not know how, and awaken naked under sources of light. The events are highly traumatizing, and no one who has experienced this bizarre situation has ever been able to pass it. Every person who survives develops expected Nyctophobia, and many of these survivors are often taken again after stepping out of light while still on a planet being consumed by Encroaching Darkness.

Once all living animals on a planet have been taken, Encroaching Darkness leaves the planet. It is possible for others to enter the planet and find it devoid of life despite the obvious architecture built by intelligent life. No traces of taken persons ever reappear on a planet that Encroaching Darkness has left.