Pure Demon

Pure Demons come from another realm. They are the originals of their species and can generally be identified through their massive Energy when compared to Mixed Demons. Before False Demons existed, the easiest way to identify them was through eyes, if they had any. Pure Demons always have pitch black sclerae as opposed to many creatures such as Humans having white ones like many creatures of this world do.

Creation

Pure Demons are born of pure Energy coming together to create a (possibly) sentient being. They can only be created in their own dimension, where the Energy is free-flowing and plenty, but there are a few exceptions to this. This Energy condenses in a pod-like structure that has no name in their realm. If a Demon was not born through manifestation within a pod, they cannot be called a Pure Demon. Because of the nature of their creation, the few who exist in this world are here only because of access to Kitasun's portal, and his permission to continue living in this realm. Pure Demons are all born with a single "instinct" that guides them in their existence:

I must kill. Killing brings power, and fear. Surrender only breeds comfort.

With this, their goals become clear from the moment of creation. Once enough Energy has come together, they emerge from within the pod. Should they be unlucky enough to be born without any limbs or no control over their powers, they will face death at the means of their own kind within moments of creation. Should they have limbs or powers to aid them in their first few moments, they begin killing whatever weaker Demons they can find in the vicinity in order to quickly grow in size as well as strength. Fresh Pure Demons spend much of their time around the pods, picking off whatever stragglers they can. Eventually they grow large enough that some wander further away from the pods in search of stronger prey, and they continue killing and eating their own kind until they meet their doom. While they are eternal beings who are not bound by time or any other mortal limits, the nature of their abilities makes it difficult for any one Pure Demon to grow infinitely large. It is possible, but many die before then.

Differences Between Pure Demons and Other Demons

Pure Demons can live forever. Pure Demons are not bound by age like Mixed and False Demons are. Mixed Demons cannot live forever due to them being a combination of animal or Human and Demon, and eternal life is not something that can be passed on from the Demon parent. If left alone, they can survive for all eternity in this world.
They can only be killed by their own kind. A False Demon cannot kill a Pure Demon, and neither can a Mixed Demon. If either of these is somehow strong enough to even injure a Pure Demon, they cannot compete with one if they were to unleash their full power. There is simply nothing as strong as a Pure Demon, especially one who grew large enough to surpass many of their own kind back in their realm.
They cannot be created naturally in this world. Mixed Demons are the result of creatures of this world breeding with Demons, and False Demons are lab-made.