Mummies are preserved corpses found in extreme climates.
While some are not the result of intentional preservation, this post will only discuss mummies that have been preserved on purpose.
When someone dies, a mummy can be preserved by removing the organs, drying the body in Natron and wrapping it. Natron is very rare and its sources are guarded jealously. Moisture and exposure to the air will cause the mummy to decay, but undisturbed they will stay in excellent condition for thousands of years. In cases where Natron is not available, salt can also be used.
Depending on the culture, a priest or mage may attend to this process, chanting, casting spells and/or performing ceremonies. No matter the culture, the purpose of this process is to allow the soul to make a journey to an afterlife (good or bad).
Mummy corpses are valued for their healing properties. An ounce of ground mummy corpse in water is the equivalent of a potion of extra healing. Mummy corpses weigh about 40 pounds, which make them incredibly valuable.
Mummies are often buried with pickled or dried organs hidden nearby. These were meant to serve the spirit in the afterlife. A pickled mummy liver can cure the most deadly poisons if eaten. The brain will either impart knowledge of the past or create a temporary connection to the mummy’s spirit in the afterlife (whichever serves the DM’s purposes). Other body parts are rumored imbue various benefits such as fertility, vigor (+1 CON) and charisma (+1 for eating the tongue of a chief or king).
Most of the remaining unplundered mummies (in my world at least) are of the undead sort. These souls did not make it to their destination for some reason. Perhaps the stars were not favorable at the time of death, the gods deemed the deceased unworthy or perhaps a priest sabotaged the process. Often such souls or spirits are not aware that they were rejected at the gates, or even that they are dead. The body has taken to wandering the gravesite or nearby environs.
The undead mummy’s soul might be trapped in a nearby object. In some cultures, the soul is placed via clerical spell in a piece of jewelry, a weapon or a mummified slave’s body or pet. In cases like this, the mummy is not dead until the soul’s container is destroyed. These containers can be dangerous—destroying an undead mummy’s body then wearing its soul amulet out of ignorance will lead to possession, for example.
Mummies have the same intelligence and personality they had in life, although the centuries in between may drive the soul mad if the tomb has been disrupted often. Activity awakens a mummy and that makes them angry and prone to attack. Whether their motives are malevolent, misunderstood or unknowable is up to the DM.
Mummies bodies have HD appropriate to their station in life and the magical/clerical capabilities of their culture. The more magic or prayers involved, or the more sacrificed slaves and animals buried with the mummy will raise its HD. An honored pharaoh’s mummy will have 10 HD or more, while the mummy of a northern barbarian chief might have 2 HD.
While the physical damage dealt by an undead mummy is not that great, its touch drains a level (a saving throw might be allowed to instead lose 1 point of CON for a year). Some mummies have other powers sometimes including: spell casting, weapon use, cause disease, cause blindness/deadness or teleporting others to afterlife plane.
If the mummy’s soul is in its body, destroying the body will destroy the soul and the mummy is gone forever. The remaining corpse might not heal the same as a regular mummy corpse. There may be strange side effects.
If the mummy’s soul is housed outside the body, the body cannot be destroyed. It might retreat or temporarily turn to dust, but it will reassembled within a day if the soul’s container is not found and destroyed via blessing, dispelling or other means.
When using a mummy in your campaign, consider removing some of the cliche trappings such as the wrappings or the Egyptian trappings (unless they are appropriate). You certainly never need to use the word ‘mummy’. It’s entirely possible that without the wrappings, they’ll think it some other generic brand of undead.
The idea of mummies bodies having curative powers is based on history. People actually believed this.