Common demons are the typical result of a summoning. Their spiritual self exists in the demon realm, brought into the world via the summoning process, which makes them take on a physical form. This physical form is "loose" and therefore difficult to harm without holy energy, as it's created and held together with demonic magic.
The physical forms of common demons vary, but are always dark in color and typically lack sensory organs. Any number and type of limb may be present, and mouths (without connection to a digestive tract) are common features as well. This physical shape, formed from pure demonic energy, often vents this energy as black smoke, especially when harmed. Holy energy is the only way to truly destroy them, but taking consistent and heavy damage can disperse the magic holding their physical form together and banish them to the demon realm until they are summoned again.
Pale green crystals serve as the connection between the demon realm and the physical world, and are an important part of the summoning process and are capable of being used to "infect" others.
Pacting is when a demon's mental and physical form melds with that of a sapient individual through a ritual the individual must perform. This alters the individual's body, causing horns, tails (in some cases, extra tails), and spikes to sprout. The demon and the individual remain as two separate consciousnesses, occupying and controlling the same body. During disagreement, this causes pain to the individual, and those who pact have two options - learn to work with their demon, or give up control completely. Most choose the latter path.
Pacting can occur with humanoid and anthropomorphic species. In Daiten, attempts to pact will result in corruption for the Daiten and banishment for the demon. In other species, a failed pact will leave the demon's physical form intact but weakened and vulnerable, and will cause a number of demonic illnesses and afflictions.
Forming both above and below the ground as a result of the planet's magical forces, jagged green crystals serve as a connection between the physical realm and that of the demons. They are used as part of the summoning process, and are required for the connection to actually raise a demon into the physical realm.
They are also capable, upon coming in contact with an individual's bloodstream, of connecting them to the demon realm. In humanoids and anthropomorphs, this will warp the person's bone structures and mind, turning them monstrous and driving them to harm others around them. Eventually, the body's energy will be used as fuel to form more crystals, which sprout directly from bone and are most prominent along the spine and joints.
Those infected with crystals are not capable of rational thought, unlike those who have pacted with demons. In Daiten, they serve as a corrupting force. Most other species are susceptible.