Mummies & Mummy Lords

Mummies & Mummy Lords are a specific type of undead that can plague the living. A mummy is the physical remains of a human or humanoid that has gone through the process of mummification. This process is either done artificially or in rare cases naturally. When done artificially it is done as part of the funerary rites. The old Egyptian Empire possessed the knowledge of doing this with their dead, and the rank of the person denoted the skill & time it took to do it correctly. Many mummies that still exist is of moderate to high ranking, with their internal organs removed and put in canopic jars. The rest of the body, and that jars contents were treated with specific herbs, salt, and chemicals to keep it in as best condition as possible. After 70-71 days the body would have been preserved, wrapped in linen sheets, put in a coffin or sarcophagus according to rank. The pharaohs, royal family members, and high ranking priests had the most extravagant burial coffins, sarcophagi, and tombs. Most of these still in existence is found in the Pyramids at Cairo, or the mythical Valley of the Kings. The lower the caste the chances are the body did not get full treatment, and a plain coffin or burial shroud to cover the body in the grave.
Most tales of mummies returning concern these being cursed individuals where magic has been used. Each time the embalming (as it is called), is carried out there is the chance that the body has spells from the Egyptian Book of the Dead, inscribed in the tombs, on the body & linen, and cast during the embalming process, during the internment, and after sealing the tomb. This allows the individual to be trapped in the body & Material Plane, and rise again under certain circumstances. These circumstances usually involve protecting the tombs and the treasure within. Sometimes, it carried out at the behest of the dead individual who requests to return because of unfinished business, or because they are not allowed into the afterlife. The sets of magic used depends on the rank, with the majority being done on the priests and lesser nobility, and mummy lords being high priests & the pharaohs. For each tomb complex there is usually only one mummy lord, and multiples lesser mummies.
Other types of mummies exist depending on circumstance. In the Far East some Buddhists carry out a ritual where they drink poisonous tea, and honey, as well as rubbing in the latter. At the same time they carry out specific mantras & meditation to focus their minds. In the case of Tibetans there is their own Book of the Dead that helps with the mediation. After a while they are placed in the lotus position in a makeshift chamber that is unsealed up to 100 days later. By that stage it either has worked or a complete failure. In the majority of cases this is carried out by Buddhist monks. Sometimes things go awry, and the soul still lingers instead of breaking the great cosmic cycle and the mummy awakens to torment the living.
A few people are mummified by other means and it usually involves extremes of environment. The Celts would carry out sacrifices to their gods by killing a well groomed and fed individual, and then place the body in a bog. The bog stops the body decaying but is acidic enough to dissolve the teeth and bones of the individual. Sometimes the body is dug back up and moved to another burial site later. A person might die in extreme cold environments causing the body to effectively freeze from the inside out, and be found later by others. A few also suffer the opposite fate, dying in a hot arid environment, and the body ends up drying out & becoming desiccated. With these types the mummy might return as a curse of some type, being risen by the likes of a necromancer, or as the undead with their souls being unable to pass on to the afterlife.