Souls

All creatures have souls. Animals, people, elementals.

Souls process energy. Typically they process positive energy. By eating food you take the energy that it needed and consume it (on a spiritual level - the regular biological processes still apply.)

There is no afterlife. Upon death, the energy making up the soul disperses into the Machine of Spirits like a bubble full of smoke. If a God existed the bubble could be maintained as an extension of belief in that god. Souls can be worshipers just as the living can be. However, given the lack of Gods in the setting, no souls exist beyond death. This means that undead such as ghosts do not manifest. Though, suitably traumatic events can cause a fraction of the soul to calve off and bind itself to something perpetually reliving the event and inflicting it upon other (see the dagger owned by Ter'ran haunted by a suicide).

Undead:
The undead also have souls. But the key difference is that their soul is inverted. A dead body wants to remain dead. The inverted soul must continually draw in energy faster than it is drained as a way to keep their soul intact. They have to fight against entropy. Unintelligent undead have very little drain, as such they can remain animated until their bodies are ground to dust. Intelligent undead are different, each form of undeath is usually unique in how the disequity is managed.

Vampires:
Vampires ingest the vital blood of others to maintain their unlife. Each drop buying them time against a true death. Given the right circumstances it is possible for their undead soul to fold back upon itself feeding off of its own entropy creating a true immortal, but it is in reality a fate worse than death.

Liches:
Liches are creatures that have achieved some manner of immortality by warping their own soul in a way that they can sustain themselves beyond death.

Arcane Lich : Machine of Shadows
The more commonly known lich. Basically an arcane caster that uses the Machine in Shadows to generate energy to keep their soul intact. They have to have a physical phylactery that serves both as the physical manifestation of their soul but also as the physical reference point for the machinery fueling them. Within the phylactery is effectively a pocket dimension mirroring the machine of shadows. Their soul is at its core. The phylactery can be destroyed one of two ways, the first is to destroy the physical phylactery the second is to go into the Machine of Shadows, find the secreted away pocket and destroy it.

Divine Lich : Faith based Divine Magic
The lich becomes an idol. They require worship, without worship they will wither away and die. This one is very difficult as the would be lich requires a form of lesser apotheosis. Their physical body is burned away in a burst of divine light and a new body is formed based on the beliefs of the religion. The new body is whatever the religion would consider an divine being, like an angel or demon. This is the most powerful of liches as they suddenly gain a great deal of power, but it is also the most unsustainable. Empowering their avatar body and abilities requires a great deal of energy. If they falter their body will implode until they are crushed down into an inanimate dormant object.

Flesh Lich : God's Blood
Think 'if a druid became a lich'. Their flesh becomes their phylactery but they are very much alive. Basically they become 'the thing'. They take cellular memory to a whole new meaning. Each cell becomes in effect its own phylactery each containing a sliver of the lich's soul. They can reshape their body at will. If flesh was destroyed by fire or acid then a part of the lich would die including some of their memories. A flesh lich can regenerate from even the smallest sliver but it is little more than an animal at that point, a thinking brain with no memories. (Technically they could survive as a single cell, but at that point they're basically an amoeba and won't form a colony). Any land that has encountered a flesh lich will be plagued by mishapen flesh beasts for generations. (Some manner of template - extra teeth, bone spikes, dire animals might even have originated from one. See Parasyte X anime for more ideas)

Psionic Lich : Machine of Spirits
The hardest to kill and the rarest. A Psionic Lich turns an idea into their phylactery such as a folk tale. However, unlike other Liches the corporeal form of a psionic lich is a manifestation of the tale. (Think of a tulpa). If the tale becomes darker and more malevolent so will they. Note, belief in the story does not matter, 'knowledge' of the story does. This is because each time a person thinks of the story a sliver of energy is stolen by the liche. If the story is told they gain far more power.

Parasite Liches: Patron Magic
A Patron Liche has multiple phylacteries in the form of undead and living hosts. They Parasite Liche carves off a specially prepared portion of their soul and implants it into another. Unlike other liches their power is increased with the number of phylacteries they create and can even regain a fully living body. They feed on the disequity between their living and undead phylacteries. However, a parasite liche can still be destroyed and will not regenerate from her phylactery without outside help. Example: Gnoll Matrons and their Husbands.

The Living Dead:
Intelligent Corporeal undead with minds intact. See Mondaleen's Dead Quarter. More common in The Widowmark where there are vast amounts of their workforce as living dead labor. The Living Dead are commonly treated as second class citizens, treated as chattel or as little more than slightly more talkative zombies. The vampire lords look down on them for their overtly undead appearance (reminding them of their true mortality is not acceptable) and are not welcomed by the populace because when you're a serf, you feel better if there's someone lower than you. It helps than any reanimated have no memories or family ties that might support them.

Only created via necromantic magic in conjunction with a Ghoul Sun