Undeath

"Silver blades, rowan stakes, bronze bells. If you're not carrying at least one, the dead'll eat you up." -Captain Nutmeg of the Grave Wardens

Mortals have - are - choices, from birth unto the end.

The end, usually, is death, which is best understood as a transformation into something new. But not all things are ready, or able, to leave their old life behind. Some obsessions are so strong they serve as an anchor to pull the soul from Death and create an undead. This soul, half in the River of Death and half in the mortal world is called a vestige. A vestige holds the personality and individuality of the specific entity. In some undead, much of the soul is left in Death and what remains is reduced to animal instinct. In others, it resembles a shadow of personality the undead had in life. However, the difference between a dead mortal and an undead is the influence of that anchor.
An incarnation or celestial has a virtue or ideal, a defining aspect that grants them power but comes with restrictions. Undead are anchored to the mortal world by a hollow, a type of negative aspect . This hollow is something the undead craves that it draws perverse power from. Typically, this is a hunger, or an fixation. As long as this hollow cannot be satisfied, the undead persists in this world, willing to do anything to get what they want.
Undead defined by hungers include zombies, ghouls and vampires among others. They crave life, in the form of meat, blood, the rush of violence or power over others. They will do anything to indulge that base instinct. Some undead simply move towards the warmth of a living being and try to feed on spiritual energy, but others can plan, bide their time, and rationalize their cravings. The longer they go without feeding, the more power the hollow gains over the vestige. They become less cunning, but more savage and dangerous. Hungry undead sometimes have strange and specific tastes in how they fill their void. Some will lick filth, swallow coins, or other strange behaviors. Most tend to, as a side effect of continuing to exist, pull the aether out of the space they travel though.
Undead defined by a fixation include ghosts, liches and revenants. Their anger at an enemy in life, failure to finish a great work, or service to a powerful master causes them to rise to finish the job. Unlike the undead of hunger, undead of fixations sometimes fulfill their hollow and pass from this world naturally. Others set a task that cannot be easily satisfied, like "being the strongest" and continue on until they are stopped. These undead tend to retain more of a vestige for longer, but frequently, their fixation brings a slowly progressive madness. Obsession blinds them to things outside of their focus.
Because undead are defined by hollow and not by restraint, a being that has been undead for any length of time tends towards antipathy towards the living. Their aspect is inherently harmful. Therefore, necromancy, the creation or command of the undead, is malfeasance. Reanimating the undead is a type of dark investiture, imposing a negative aspect on a dead soul to trap them in the mortal world. After robbing them of spiritual completeness, a necromancer can use this to control the undead by feeding aether into the hollow, enough to dull the pain but not end it. Necromancers who reach an agreement with the spirit before reanimation are morally dubious, though freeing the spirit after its service is mitigating. Many cultures produce exorcists and crusaders to combat this practice. Unlike incarnations, undead do not always need a physical body. Some have a hollow so strong they can pull aether into their aura to allow it to affect the world directly. These bodiless undead are notorious for moving though walls, ignoring weapons, and vanishing at will. Nonetheless, if their aura can be attacked, or a corporeal undead is damaged, their hollow can be separated from their vestige and destroyed.
Exorcists and other people who oppose the undead do so because of the danger of an uncontrolled hollow. Many religious sects believe the state of being a vestige to be sinful or at least pitiable. Undead are not just a danger to mortals, but to themselves, as their hollow torments them and drives them to add more sins to their souls. Exorcists typically seek to send the undead back to death and the subsequent afterlife, by either filling that hollow or by force. Ghosts and revenants who typically desire something tangible can sometimes be granted that last wish to free them. Others must simply be rendered unable to continue. Undead that steal life force sometimes perish through starvation, unable to maintain their anchor. Others can be rendered unable to feed through decapitation, burning, staking and similar methods. Sunlight and other forces with aspects of cleansing or purity like salt or silver degrade the bond between vestige and hollow. Sacred objects and actions can be similar, serving as an anathema and severing the connection even faster. Running water reminds the vestige of the river of Death and thus undead struggle to cross without being weakened or severed.
Some undead have as their unfinished task some kind of guardianship or advice. Undead who return to protect something typically exist between the material world and the spirit world as temporary visitors rather than trapped, destructive souls. Though like all undead, a guardian spirit will stop at nothing to achieve a task, the selfless nature of that task prevents them from vestige degradation or the excesses of malice. They return, defend, and go back into the spirit world. They have much in common with the usual undead, but their ability to simply leave the mortal world prevents them from drawing the ire of holy warriors.