Mdtryhht

The limitation on there being no souls puts a stopper on the most of the typical necromancy stories. Most of them rely on there either being a very specific soul that retains the connection after death, or the affinity of a human-shaped body to a human-like spirit.

If your necromancy is something from a Frankenstein story and you need it to sound scientifically plausible, then, as I've read, brain cells retain the ability to react to stimuli 8 hours after death. So, if a necromancer raises the body in this time window, or has a magical ability to preserve the brain longer then that, he can raise the body. If reanimation restarts the electrical activity in the body, then effectively you have a creature with massive whole-body gangrene. I can't say, how long will the resulting zombie be functional, but not for long, unless necromancer has some additional tricks to help (and if he can stop or reverse gangrene, why is he an evil necromancer and not a healer?).

If there are no human souls, but there are some kinds of ethereal creatures, spirits it daemons, they may be coaxed and bound into the human-shaped body - then you effectively may have no limit on reanimation time - a skeleton would serve as well, although it will not have the memories of the original inhabitant of a body (to access the memories you still would need 8-hour limit).
In the second case a golem could serve about just as well as a real corpse, though.

To answer your question, i will tell you about my own science-based magic system and how necromancy works in that.

My system treats magical energy as another type of energy, such as thermal or kinetic energy. In order to cast a spell, you convert magical energy into another type (or several types) of energy.

Kinetic Energy

For necromancy, you would convert magical energy into kinetic energy, thusly moving the corpse. In this way, you are not really reviving the corpse in the sense of bringing it back to life, instead you are manipulating it, similar to a puppet master pulling its strings. It is far more accurate to call this reanimation rather than resurrection.

In order to prevent the corpse from decomposing, i use what i call “kinetic barriers”. These are areas of kinetic energy which act like a solid object. Essentially, any force put upon this barrier is resisted as the kinetic barrier pushes against the force put upon it, thusly cancelling it out. In terms of a corpse, the kinetic barrier covers its body and pushes away oxygen and pathogens, preventing the corpse from decomposing. This means that, as long as the spell lasts, the corpse will not decompose and will be able to move.

Electrical Energy

Due to how necromancy works in my world, it means you can’t bring someone back from the dead (unless using the chemical energy method described below). You could control their body but not bring them back to life.

Instead, to somewhat overcome this, i though of using electrical energy much like how a defibrillator is used. In this way, you could ‘revive’ someone by stopping their heart with electricity, allowing it to restart. Whilst this wouldnt work in all cases, it would in the case of a heart attack which could cause someone to ‘die’ (in reality, we know they’re not dead, just suffering from a heart attack. However, medieval people might not realise this and assume the person was brought back to life).

This method allows the revived person to remember most, if not all, of what they did before they ‘died’, prevents them from decomposing and makes sure their biology is exactly the same. However, it isn’t true necromancy, though people may see it as such.

Chemical Energy

This method is a lot harder to perform (and harder to describe accurately). Essentially, healing magic in my world works by converting magical energy into chemical energy and using that to accelerate the body’s natural healing processes.

Applying this to necromancy, you may be able to repair the damaged cells in the deceased’s body using chemical energy. This would cause any internal injuries to be healed and the body to be refilled with blood. In this case, it is perfectly reasonable to call this method resurrection as you are actually bringing someone who died back to life.

There are several issues with this method, though this plays perfectly into your question. Firstly, the body would need resources to repair itself, such as with proteins obtained from food. Second, if i remember correctly from my CPR training, it only takes around 3-5 minutes without oxygen to get brain damage. This would mean your mage needs to be relatively quick if they want to keep the subject’s memories perfectly intact. Otherwise they might have amnesia or partial amnesia. Finally, if the spell works, the person is no longer dead, they are as alive as they were before they died, meaning they still need to eat and breathe and perform every other bodily function the human body does.

Bonus - Where to get the bodies

When most people think of necromancy, they think of necromancers raising the corpses of deceased solider on a battlefield or raiding graveyards and crypts. However, there are so much more interesting things you could do.

For example, executions. A necromancer might go to a prison and ask the guard if there are any prisoners awaiting execution and, if so, may ask to buy the body. Obviously this would not work as well if they were beheaded, though if they were hung or poisoned you could easily reanimate the body. (Also, a guard may say ‘yes’ and execute a prisoner who wasn’t on the list to get a bit of extra money).

An alternative may be to go to a hospital morgue or a holy site, though they might be more reluctant to hand over the deceased. A thieves guild, mercenary guild or an assassins guild may quite readily hand over bodies though. You could also pay grave robbers to bring you bodies, much like what was done in Victorian London where scientists were dissecting human cadavers to find out how they worked, though this was highly illegal.

Another example would be contracts. Local peasantry or the homeless are famously short on money, something an enterprising necromancer may exploit. A necromancer may go to a local peasant and offer to buy their body once they die, paying them in advance. Whilst this might seem ludicrous and you may think no one would ever do this, you have to remember this peasant may have a large, starving family and no way of paying for or making food. This works especially well in times of famine or war or simply when taxes are raised significantly. (An evil necromancer may pay someone, have them sign the contract and kill them, this works best if if looks like an accident and does not happen immediately after they sign the contract. If the local law enforcement come, the necromancer has the contract proving that the victim’s body belongs to them.)

When someone comes along with a bag of coin offering to buy their body, which they wont be using once they die anyway, it may seem a tempting offer. Its a similar idea to how people donate their bodies to science in our modern world. You have even have this on a larger scale with magical academies making these offers and giving the bodies to their necromancy students.

The magic has the power to recompose. The more decomposed the body is the more magic it requires to recompose a body. Thus stamina and skill of the necromancer may play a role in how decomposed a body one is able to revive.

If the recomposed body requires constant addition of magic The expiration date is when the necromancer runs out of stamina or for some other reason cannot maintain the spell because of losing concentration, sleeping, dying. Concentration could be maintained while resting in meditation or necromancers could have the skill to not sleep (this would be unhealthy) to allow longer periods of use.

If the revived one is perfectly brought back: The dead did die for a reason. That reason might kill the revived one quite fast.

Necromancers are doctors.

If you have a society that views any medical intervention as "magical" or "unnatural", then doctors are effectively using magic to prolong life unnaturally, especially, if you consider interventions such as life-support.

You could approach this in one of two ways

(1) the doctor is benevolent, saving lives, but as a result of social beliefs, those saved are outcast, and the doctor is viewed by society as an evil necromancer.

(2) the doctor is manipulative, using medicine to control their victims. This is reminiscent of existing theories about haitian zombies being the result of drug cocktails which induce an artificial coma followed by a hallucinogen which would make them very susceptible to the instructions of the person who had administered the cocktail to the zombie. (Wikipedia)

The only caveat here is that the person never died under our definition in the first place. However, the legal/medical definition of death has changed over time. For example, a lack of pulse used to be a standard definition, but with modern medicine, this definition is no longer adequate. Let your society subscribe to an older definition, like pulse, and be suspicious of the apparent re-vivication of those whose hearts had previously stopped.

Your question specifies no smell or decay, but if you wanted the "dead" to decay, consider medical treatments that do not cure disease, but only slow its progression.

Resident demon.

Your reanimated dead are reanimated thru placement of a summoned spirit. This spirit moves into the body and maintains it. It will repair what it can. It occupies the brain and can derive some memories and other things from what is left.

The spirit can only do so much. A body might be too far gone with decay, which will piss the spirit off.

If you have substandard corpses, or want to make something better, it is possible to augment the corpse, or fortify it, or combine good pieces from several corpses either before or after the spirit moves in and takes over.

"Realistic" necromancy would be restoring all the basic functions of the body, which would include most organs and tissues found in a living body. So no zombies that don't need to breathe, eat or whose only essential component is the brain.
I would argue that the end result would be a mostly normal human being, perhaps one that would rapidly deteriorate without frequent maintenance from the necromancer by way of manually tuning the various organs, maybe because once returned to life the body doesn't really "know" how to properly maintain itself in the long run.

I've always assumed necromancy to be the (temporary) animation of dead bodies. This would perfectly fit your requirements.

• 
  

  A dead body is just a bunch of bones and organs in a skin. Through the use of magic this mass is given a way to 'defy' gravity, to have a certain amount of force, and (if so desired) is animated to resemble former behavioral modes.
  
  A good illustration of this would be the armature used for rigging 3d models to animate them:

  
  
  

  

  
  
  

  Magical bones are added to the dead body, that can change position within (or, more gruesomely, without) the limitations of the body.

  
  


• 
  

  Considering you seem to want the dead to retain (parts of) their memory, this will be decisive for the period of necromantic reanimation: the longer the magic practitioner waits, the less brain cells will be intact, and that limit is around three minutes:

  
  
  

  
  

  The brain, however, appears to accumulate ischemic injury faster than
  any other organ. Without special treatment after circulation is
  restarted, full recovery of the brain after more than 3 minutes of
  clinical death at normal body temperature is rare ^{source,
  but?!}

  
  

  
  
  

  How one would tap into those brain functions is ..magic!

  
  



• The expiration date of the animated 'meatbag' would be either that of the physical deteriorating body (slowly falling apart), or, (un)naturally, its deterioration decelerated by magic, shielding the body from external forces and internal detrimental processes.

Given what you said about not having souls and such, the only way Necromancy will work (or at least be evil) is if it's the more gruesome mindless kind.

Let me explain, in a society with absolutely no chance of an after life, there would be a great drive to prolong ones life as long as possible. Even without that, there would be the same drive to preserve ones life in the case of illness or injury. This is only natural behaviour but without the comfort of religions and afterlife gives.

So if your Necromancers can preserve the mind and the body of the dead, they would of course be highly valued by such a society. Which obviously makes them non-evil.

Now however if they cannot preserve the body or the mind, well that's a different story.