This is separate from the Necromancer build used by some Summoners, though some who run said build do acquire this subclass for bonus perks. It is also unrelated to the Specter User subclass despite having similar purposes.
The subclass can be acquired by acquiring some books on necromancy and then go through a series of quests deciphering them. Though there is technically no other prerequisite for the subclass, deciphering texts is a skill relegated to a handful of subclasses, such as the Scholar. As a result, it is considered by fans to be an extension of the Scholar subclass. The quests themselves are rather difficult, and some require the player to be above a certain main class level, so all-in-all it's a difficult subclass to attain.
Because summoning creatures to aid in combat is an exclusive ability for certain main classes, undead creatures summoned by Necromancers act more like tamed beasts from the Animal Trainer subclass, following behind like pets. At higher levels, some undead creatures can be used as mounts. For the most part, however, this is mostly for role-playing and provides less practical effects. Combined with its acquisition difficulty, the Necromancer subclass remains a less popular subclass relegated to a few dedicated role-players.
Some Summoners who run the Necromancer build will acquire this subclass due to gaining certain bonuses and synergy for the build.
Legend has it that a famous mage tainted himself with forbidden arcane arts to acquire this ability, and he is mentioned in the subclass quests related to this subclass. Naturally, this isn't an occupation available to People of the Land, and the few who do know this art make sure to conceal and protect its existence.
There are no known Necromancers at this time.