Kreptis
A kreptis is a being entirely of shadow, and is never usually solid. They have a slight ability to solidify, but without any sort of density. Kreptis have a very thin surface, but applying pressure would cause it to give quickly and phase right through. These beings usually take on the forms of animals or mythical creatures in shape, but the form is featureless, shadowy smoke billowing off their silhouettes. Inside the kreptis's shadow-frame lies a core. The very soul, essence, and energy of the kreptis is in this core. It too is relatively formless, spherical, but encased and protected by the kreptis's shadow-form. This core glows and emanates from their black shapes in various markings, the color depending on the type of soul it harbors. The intensity of the core and the size of the Kreptis depends on the strength and power of its essence, and of course its darkness. They are able to change, stretch and grow their forms to a certain extent, but this depends on the capacity and strength of the essence in their core. Weaker Kreptis cannot shift or stretch for long without exhausting themselves. Certain kreptis are born into their world with abilities, or limited powers. It is believed that the past life of the kreptis held a connection to these abilities. Such as if a Healer or Nurse had a talent with healing, a kreptis might be formed with limited abilities to heal. An example in that nature would be something like the healer kreptis having the power to use their tendrils to mend wounds, or even transfer mortal soul/energy essence they had consumed to others. Other abilities would be similar in simplicity. One kreptis might be more in-tune to the living world, able to interact a little more with mortals than a typical kreptis. Another might have a stronger, warrior's core, and possibly larger fangs or claws if they were some sort of knight/fighter in their past life. Keep in mind that while the kreptis wake with these abilities, they are quite limited and any abilities require the use of core essence, their basic life force. So things like controlling another's mind completely or having total weather control abilities would be too much for a Kreptis core to handle. These powers are subtle and connected to Kreptis life energy, and should be noted as such.
A kreptis battle would be like watching masses of shadows clashing, thrashing into each other, their forms meshing and mingling. You could picture two shadow wolves fighting each other and actually connecting with one another as if they were solid. Much of it depends on the one with a stronger essence. Imagine the shadow wolf biting the other one's leg. You can picture some resistance given, but it will eventually "snap" off or vanish only to re-materialize. But the more a kreptis materializes, the more the opposing one is eating away at its opponent's core's source. They'll slowly dwindle and fade. Very similar to exhausting it to death, depleting it of all its essence until it is gone. While kreptis have the ability to "fight" one another, a living being cannot exactly touch a kreptis, and physical contact with one is extremely limited. Hurting a kreptis would be impossible, unless light was involved. Light is a shadow's worst enemy, and a kreptis is no exception. All kreptis avoid daylight. The light eats away at the kreptis's shadowy form, and leaves the core unprotected and eventually it fizzles out as soon as a matter of minutes. All kreptis possess shadow tendrils in which to feed. They act as sapping appendages, drawing out and absorbing the soul energy/essence of living things to keep their own essences strong and healthy. All kreptis must feed to maintain their forms, any who do not feed slowly lose their essences; cores fading and fizzing out until eventually they die. A kreptis can typically make themselves seen to living beings as they wish, including during feeding. Typically though they are only seen as dense shadows in the corner or dark shapes that flit from your vision. While possible to feed enough on living beings to kill them, it would take multiple kreptis to do so. And if done, the victim turns into a kreptis when they pass, falling prey to a scarily traumatic death. Many of the species consider this unjust and wrong, as the life of a kreptis stuck to wander the middle realm is an ugly fate. Many don't wish the same destiny for other living beings. Kreptis are physically capable of feeding on each other. But this practice is incredibly taboo. This is because the result of a kreptis "stealing" another kreptis's essence may cause an unwanted or even dangerous response. If the two essences don't match and mesh well, it could possibly cancel out both essences and kill them both. There's also a possibility one essence might match, but one is stronger and the lesser dies. Legend says that there are some essences that mesh and bind so well that they forcefully merge into one, whole new kreptis.