Necromancer

A necromancer is a creature that learns the secrets of life and death, becomes immortal and is able to raise corpses as undead.

Necromancers sometimes also do necromantic experiments on the living, usually on citizens of conquered settlements and this results in the creation of new races, unique to each world.

Necromancers can learn their craft in two ways: either becoming obssessed with their own mortality and attempting to cheat it, starting worshiping a god of death (if they weren't already) and said god giving them a slab with the knowledge they need for their apotheosis, or being taught by a necromancer.

Adventurers have a third (and their single) option: to read a slab or a scroll containing these secrets. The adventurer has no choice but be imbued with immortality and the ability to raise the dead. However, the adventurer can abstain from raising bodies for the entirety of his player-controlled life, as raising undead is an entire voluntary action.

There is no known way of removing these abilities in vanilla DF, however via DFHack, it may be possible to do so using this technique. Alternatively, using DFHack's full-heal command will remove the ability to raise the dead (though the character remains referred to as a Necromancer in legends mode), as the necromantic abilities are granted by syndrome.

Tools
A necromancer's tools (during World Generation) are multiple:
 * Raise undead.
 * Their necromancer group. Until evidence to the contrary, once a necromancer loses their group, they can/do not make another one, but drift around the world until they are slayed.
 * Their Tower: a place to store artifacts and a stronghold for emergencies.
 * Artifacts: Usually when a necromancer declares war on a civilization and is successful, one or more artifacts may be given to them, sometimes massively increasing their personal power.

Mishap
It is entirely possible for the rise of a necromancer to be cut off in the crib.
 * An aspiring necromancer could become obsessed with their own immortality... and nothing will happen. The god of death they worship will not respond, or will not respond in time, and the aspirant will either be slain before a god answers (if it was ever going to answer) or simply die of old age.
 * A necromancer will receive their slab, will learn the secrets of life and death, will continue to work into their society until they are discovered to be unaging, where they usually flee... except the latter doesn't happen and they get killed.
 * A necromancer will learn the secrets of life and death, build themselves a tower... and be overwhelmed by an opponent. Their group will be destroyed, but it is entirely possible for the necromancer to escape with their life, but they will be taken prisoner and integrated into the society that defeated them. Interestingly, the necromancer's captives will not notice that their prisoner is a necromancer, even if they should be famous, so in a few years or decade, they will be observed to not start aging and they will flee. They will however not make another tower, but run to another town to continue this cycle until they are slain. Examples: Genam Gigolupu, Kanil Xemisdi
 * A necromancer may be slain, leaving their group immediately. They may then be resurrected... and no longer be recognized by their group. Example: Oddom Thobmunèst.
 * A necromancer may be on errands while their group is destroyed. Upon the group's destruction, the necromancer may revert to the actions of a rampaging beast. Example: Zefon Avuztosid.

Terminology
Because the game currently does not have terminology for some necromancer interaction, these will be the terms used for such:
 * Chosen necromancer: necromancers who were directly and for their personal use given an immortality slab by a god of death
 * Chosen necromancer lines: a necromancer from this category is one that was directly taught by a chosen necromancer or someone who was apprenticed under the Chosen or under someone under the chosen or so on.