Necromancy is the ability to summon and control things that are dead, or other types of creatures. A sub version of this is reanimation, which differs from Necromancy, as rather than controlling ghosts, is aimed at zombies.
It's unknown who and when which early found this art. It's also unknown when humans became aware of and began to learn this ability. However, the earliest known (if not the earliest) ritual for this purpose has been known and recorded since Ancient Greek era.
Samhain - Samhain, the origin of Halloween, can summon ghosts, monsters, and even demons (of lesser power), to do his will. As Sam, an experienced hunter puts it, he, essentially, can eventually control everything he and Dean ever fought (as of the episode of "It's the Great Pumpkin, Sam Winchester").
Osiris - Osiris, the Egyptian god of death, could summon ghosts and cause them to do as he commands.
Death - When he, under the instruction of Lucifer, resurrected people, they became zombies soon after.
Colonel Eckhart - A Thule Leader. He was a strong necromancer, being able to automatically revive himself if he is killed and not age, though burning his body will kill him permanently.
Torvald - A Thule, who has the same powers as Eckhart.
Nauhaus - A Thule who had the same powers as Eckhart.
Harper Sayles - Hailing from a family of necromancers, she brought her boyfriend back to life and had him feed on would be suitors to maintain his form.
Note
The Thules practiced, experimented and attempted to perfect necromancy to the point that their subjects were not simply enslaved or summoned spirits or mindless zombies, but actual resurrected humans. It may also be possible that their practice was a misnomer, and that they were actually working on the secrets of resurrection, as opposed to necromancy. Although it should be noted they didn't come back as completely natural humans, as now they could only die if shot in the head and then had the body burned.
