When the Gods destroyed the first world only a few creatures were able to survive and escape to become the Fey. But that does not mean the ruins of the first world could not sustain life. From the magic and divine spark of the dead Gods arose the three: The Demiurge, The Aeon, and Deus. As Aeon came to hate all of existence it could not act directly without being opposed by the Demiurge, Deus, or the Gods. Thus to support it's machinations the Aeon created Infernals.
Infernals are crafted by Aeon to a purpose even if that purpose is madness. Thus an Infernal of one type will look the same as another of it's kind.
Genetics and Reproduction
Infernals are made by Aeon who crafts countless copies with a sweep of its hand. A few more subtle Infernal creatures will also create lesser Infernals from the souls they make pacts with.
Growth Rate & Stages
Infernals are created at full size and all types are immediately aware of their purpose and given such knowledge to assist their efforts. Anything outside of this is alien to an Infernal but few are independent enough to have a personality or tell.
Ecology and Habitats
Infernals exist almost exclusively within the Inferno, home of their deity Aeon and their kind. A few more subtle Infernals have managed to infiltrate the Second World while others are summoned and given free reign to wreck havoc until they are destroyed.
Dietary Needs and Habits
Infernals have the corrupted divine spark that sustains their bodies indefinitely.
Infernals are created upon the Forge of Chaos and exist as they were made until they are destroyed. There have been a few instances of Infernal's passing a part of their being through the Infernal Taint but those mortals who live with the taint soon perish and are reborn as Infernals.
Perception and Sensory Capabilities
Infernals have heightened senses as well as specific senses dependent on their type. Thus a Corrupter will be able to sense innocence while a Hordspawn will sense all living creatures within a certain distance.
Civilization and Culture
Infernals have no name but are often given one in rituals used to summon them.
Infernals have no organization or factions they are sapient in that they can react to changes but their entire existence is in whatever purpose they were crafted for. A few Infernals have learned and begun to develop personalities but are twisted by their actions.
Major Language Groups and Dialects
What Infernals that are created to speak with mortals are given all the languages needed to understand but only will speak in Infernal.