Demon

Demons are malevolent spirits that exist in various hell dimensions, but can travel to the mortal realm either on their own power or by being summoned, typically by a necromancer.
Demon Summoning, Banishment, and Exorcism:

• Every demon, even the Soldiers of Hell, has its own specific summoning and banishing ritual - the more powerful the demon or the higher in the demonic hierarchy it is, the more complex and difficult these rituals are.
• Some witches are able to perform these rituals with varying degrees of success, but necromancers are usually able to perform most rituals perfectly (providing they have access to the ritual)
• A necromancer will summon the demon into a Binding Circle, usually drawn on the ground in chalk. Binding circles confine the demon and its powers to within the circle, but must be drawn perfectly or else will be useless, allowing the demon to escape. Even when drawn perfectly, if a demon summons a storm within the circle it could blow or wash the chalk away. Even the tiniest flaw in the circle will set the demon free.
• Summoning rituals involve a human acting as a willing vessel - the only reason to summon a demon is for information, and a demon in their red smoke form cannot communicate. Some morally questionable necromancers have been known to use unwilling humans as a vessel.
• Once summoned, a demon can only be sent back to hell with its specific banishing ritual.
• Generic summoning rituals exist, but the necromancer has no control over what demon it summons when doing this - and they must find the demon's correct banishing ritual to be rid of it. No generic banishing ritual exists.
• A witch or necromancer can banish a demon that is possessing a human, but only if they disable the demon long enough to perform the ritual. More likely, they will exorcise the demon and trap it in its red smoke form inside a binding circle, giving them time to perform the banishing ritual.
• Exorcism rituals are much faster to perform than banishing rituals, but only pull the demon out of the human vessel - if the banishment is not performed straight away, the demon will escape to possess a new vessel. Every necromancer family has their own version of exorcism, and all swear that theirs is the best.
• In some rare instances, if the vessel is strong enough and the demon weak enough, a vessel can overpower the demon and eject them from their body.
  

  Any living being can be a vessel, but demons will usually choose mortals over supernaturals, as supernaturals are better able to fight off the demonic possession.
When demons are summoned, usually for information or a favour, the demon will never give this away for free. They will always have a bargain with the summoner, forcing them into a deal. The most common cost is the summoner's soul e.g. selling your soul for riches, power, health, magic - in this instance a demon typically offers ten years for the human to enjoy the benefit, before the demon comes calling and collects their soul, taking it back to the hell dimension. The more human souls a demon owns, the more powerful they become and the higher they can move up the demonic hierarchy. Other things demons will accept as currency are specific memories, blood, the summoner's own magic, and procreating a child. One witch even bargained away their family grimoire in order to save the life of their child.
Demons can only be killed a more powerful demon. Witches and necromancers have only ever been able to banish a demon back to its hell dimension, never to actually kill it.