Twilight is the natural state of beings composed of ephemera in the Material Realm. Beings and objects in Twilight cannot be seen, heard or otherwise detected by mundane senses, though such beings may have the ability to affect the material world.
Beings who exist in Twilight include ghosts, spirits who have crossed over from the Shadow, and disembodied consciousnesses (e.g. the minds of those with the power of "astral projection", though they generally do not possess ephemeral bodies and may only observe, not interact with, other Twilight entities). Objects may also leave behind ephemeral echoes which exist in Twilight; these eventually decay, however, and are not alive as a spiritual reflection in the Shadow would be. Souls which have been detached from their owners also exist in Twilight.
Ghosts exist naturally in Twilight, but spirits find it extremely taxing to remain in the Material Realm, and must find a source of essence or possess a mortal host to avoid returning to the Shadow. Such possessed mortals are known as the Ridden. Ghosts may also learn the power of possession, but are unlikely to use it for extended periods. Spirits may also possess the power to materialise, transforming their ephemeral bodies into normal matter to exist in the real world, but this consumes a great deal of essence and cannot be sustained for long. By contrast, ghosts are naturally able to manifest themselves in the physical world, appearing to humans, though they remain composed of intangible ephemeral matter and must employ Numen to affect material objects.