Demons are incorporate, malicious spirits from the depths of hell. They require a human form in order to pass into the mortal realm. A dark sorcerer can sacrifice an individual in order to trap a demon in the victims body, enslaving the spirit to their will. This is a risky process that can come back to harm the user if the demon isn't bound correctly. 

Ancient runes and sigils must be carved into a living victim's skin. This will bind the demon to the body, trapping it and subjecting it to the sorcerer's will. Then the victim is killed through a dark ritual. The organs are removed from the body, as well as the bones, cartilage, etc, until only the skin is left. It is then placed over a ritual circle, while the sorcerer chants an ancient incantation that opens the gates to the netherworld. The circle provides the passageway, splitting the ground and allowing the demon to inhabit the skin of the victim. The runes keep the demon in the skin of the victim, trapping it similar to a prison.

Since demons are just bodiless spirits, why would a victim's insides have to be removed in order for the demon to possess them?
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Because they have to be removed.

This is a magic system that you're creating. By definition, the rules are also yours to define. 

**Why do you need a ritual circle?** 

Because demons only arrive via ritual circle.

**Why do victims have to be hollowed out like a pumpkin?** 

Because when they tried it on someone who still had their insides, their spleen exploded, killing the victim and three practitioners.

**Why can't we at least keep their skin on? Flaying someone is messy work.** 

Because when a victim had their skin on, the demon couldn't secure a foothold inside them and was dragged back to the nether regions, and a bunch of expensive reagents were wasted.
**Basic hygiene**

Have you any idea what happens to a dead body if you just leave it lying around. It will stink, get infested with maggots, the internal organs may explode, excrement will be expelled, etc.

Obviously just using the skin (and preferably using some form of preservative) will avoid all this disgusting mess. Sure demons are horrible and scary but they are not zombies.