Heart's Fire

Heart’s Fire is present in every person, animal or plant that’s alive. Heart’s Fire goes away when a creature dies.
There is a tiny lingering spark on dead creatures as long as the bones retain some flesh and are relatively intact This is how the low level necromancy spells Speak with Dead (which is pretty literal) and Death Sight (which lets a necromancer relive a corpses death, great for homicide detectives).
A lot of undead and lesser Void Demons are practically blind to regular sight, but their senses are attuned to heart’s fire. That’s why lesser Void Demons are very good at tracking living prey over great distances, but they often trip over rocks or dead logs.
Living creatures are usually blind to sensing a heart’s fire but relatively simple magic can give pa person the ability to sense heart’s fire or very specific variants of heart’s fire. If a magical diviner casts a spell to detect living things they are temporarily adopting life sight to find a specific variety of heart’s fire such as spells that detect humanoids, spells that detect animals, and spells that find edible plants. Even spells that detect poison see heart’s fire by detecting distortions in the heart’s fire that poisons represent.
Healing magic is essentially flooding a person with so much heart’s fire that their wounds heal. Purification Magic removes toxins or impurities from a living creature’s heart’s fire.
Dead things have no heart’s fire. Free willed undead need to steal Heart’s fire to function. Vampires drink blood, ghouls eat flesh, wights drain physical vitality. Essentially they are all different means to steal heart’s fire. Because of their undead nature, most undead cannot handle pure heart’s fire, that's why most undead are harmed by healing magic.
Void Demons were specifically spawned to feed on the living and do nothing else, so they can drain heart’s fire more efficiently.
When a spell-caster is using healing or purification magic to smite or repel and undead creature, this is essentially what spell-casters do when they use their magic to smite or repel undead. This same magic can be used against Void Demons but it’s much harder. This is because Void Demons can so efficiently feed on Heart’s Fire and a healing magic attack is essentially trying overwhelm a creature with pure heart’s fire. An attack that will blast a skeleton to dust will give most Void Demons a mild rash.

Living creatures generate their own Heart’s Fire (when they eat, sleep, and drink) and undead creatures have to steal it. Spirits borrow Heart’s Fire. Like mortals, the Nine generate Heart’s Fire themselves. They actually generate a lot of Heart’s Fire. They divvy out to their spirit minions. Because they get their Heart’s Fire from the Nine, they don’t have to eat, sleep or drink.
Elementals don’t eat, sleep, or drink. They don’t make their own Heart’s Fire and they don’t really need it. They are not alive in the same sense other creatures are alive.

Heart's Fire and Fair Folk
Most of my Fair Folk lore is still up in the air.
I’m not yet sure how Fair Folk fit into the equation. The following paragraph is just my current working theory that I could easily rewrite. Fair Folk, do eat, sleep, and drink much as mortals do, but they are sort of a hybrid between mortal creatures and spirits. They need something from mortals, I’m not sure what, but it might as well be Heart’s Fire. Fair Folk are kind of a like a hybrid between mortals and spirits. Fair Folk can survive on their own innate Heart’s Fire, but they cannot live on their own Heart’s Fire. If they want to live comfortable and exercise their magical powers they need Heart’s Fire from mortals. Which they get from mortals via mystical pacts or from proverbial osmosis. To get sustenance from proverbial osmosis they need to stir up strong emotions, love, lust, fear, hate, jealousy, etc.

Type

Metaphysical, Supernatural