I've worked on and off for a while now on an undead race for D&D 5e, so I thought it might be fun to codify how I see undead working, which can explain things like why blindfolding a skeleton actually blinds them when they don't have eyes, or whether or not a skeleton is capable of eating and drinking.

1st Law of Undead Physiology - It is extremely difficult for a ghost to physically manifest.

When you think of a ghost, you usually picture that spooky, partially transparent figure that can fly and pass through walls. It is really hard for a ghost to do that, because they're basically creating an entire body out of nothing through sheer force of will. Some ghosts can do it, many cannot. These types of ghosts are rare because they need an especially strong will. This doesn't necessarily make them any more powerful, though, as much of their power goes into simply manifesting in the first place.

2nd Law of Undead Physiology - It is easier for a ghost to physically interact with persons, places, or things that were important or familiar to them in life.

This is often why a ghost haunts a specific location, person, or object. Something to note is that this doesn't just refer to touching or moving things. Sight is also a type of physical interaction; a ghost without a body also does not have eyes, and therefore cannot see unless they manifest eyes. It's easier for a ghost to sense the things that are important or familiar to them than it is to truly see. For example, you could have a ghost who haunts a house it used to live in. Someone buys the house and moves in, and of course they begin moving the furniture around. The ghost is too weak to manifest eyes, so it is actually totally unaware of the new tenant, but it is aware that its things are being moved around, and so moves them back.

3rd Law of Undead Physiology - Undead are fundamentally a ghost possessing a corpse, usually their own.

As per the 1st law, it's often too hard to manifest a totally new body, so as per the 2nd law it's usually easier just to possess your own corpse and use that, if it's available. While in theory the undead spirit should be able to leave their corpse to go elsewhere as a ghost and then return at any time, for unknown reasons a lot of undead spirits seem to bond permanently to their corpses. Perhaps the spirit is simply too weak to manifest apart from their body, and since they spend all of their time in their body, they never grow stronger in that regard.

4th Law of Undead Physiology - If an undead is lacking in body parts or functions, they will manifest what's missing.

A fresh corpse is still mostly functional, so very little is needed on the part of the undead spirit to make it function properly. A skeleton, on the other hand, is missing everything except the basic physical frame. Even so, it's easier for the undead spirit to use the skeleton as scaffolding on which to manifest the most minimal ghostly body they can get away with. The skeleton has no muscles, so the spirit must manifest muscles that will move the skeleton. But only the minimum effort is used, as such the ghostly muscles aren't visible and don't interact with anything except the skeleton. Eyes are manifested in the eye sockets of the skull, but again only a minimal effort is used so the eyes themselves aren't visible. It's a lot more difficult to manifest body parts the creature never had, or to manifest them in an arrangement the creature never had, so manifesting eyes outside of the skull so as to bypass something like a blindfold is often simply too difficult.

A skeleton also needs to manifest an entire speech apparatus, if it intends to speak. This is especially difficult because this manifestation is not just interacting with the skeleton, but with the air which must move to create the speech. Similarly, a skeleton can manifest a stomach to hold food and drink, but such manifestations can be difficult to maintain, particularly if the skeleton does not eat or drink often. As such, skeletons are sometimes "leaky" when they eat or drink, and it varies from skeleton to skeleton. Again, these manifestations are all invisible and interact with nothing except that which they must in order to function.

Anything a ghost could do, a skeleton or rotten zombie could also do, and easier because they don't need to manifest the entire body, only the things that are missing. The force of will required to manifest almost seems to work like muscles that need exercise to become stronger, which might be why each undead always only ever seems to have just enough willpower to manifest exactly what they need. A zombie who gets an arm chopped off won't suddenly manifest a ghostly arm as a replacement, for example, but if they've always been missing parts of their flesh then they will manifest those missing pieces. An undead who begins as a corpse and gradually decays into a skeleton is continually pushing themselves to their limit manifesting each new piece that decays and falls off, until they've built up the willpower to manifest enough to control a skeleton.

Anyway, how do these laws sound? Are there any tweaks you'd make to the wording to make things more clear? Is this how you see undead working in your setting? Let me know.