Necromancy is by definition the manipulation of life energies.

This includes stuff like human sacrifice, which would technically make incidents like the summoning of Titan using living sacrifices which led to that kobold child losing it a type of necromantic ritual. Or the whole summoning of Shinryu, because human sacrifice was kind of a big thing there.

Necromancy and Primal summoning work well together it turns out, due to the whole ritualistic sacrifice deal.

And then there is what happened to the Scions, though it was an accident, getting soul yoinked like that does count. Oopsie.

It's not just to do with the undead, that sort of thing also falls under the necromantic banner if you take into account what the word means.

However, the public perception has come to deviate rather drastically from the meaning of the word, so undeath is the only thing most think about when they hear about necromancy, even though that is not remotely the only thing that particular school of magic can do.

Heck, Energy Drain is technically necromancy due to it involving leeching energy from the target, same deal for the Drain spell. Not that anyone ever thinks about that particular technicality. Nobody goes shouting "NECROMANCER!" as soon as some Thaumaturge uses Drain, even though it does technically fall under the banner of Necromancy.
Vampiric properties? That's a necromancy spell fellas.

So uhh, Arcanists and Thaumaturges kinda already dipped our toes into the outer edges of the necromantic magic school. And it may be for the best to not go much deeper than that.

Simply put, Necromancy is magic that messes with souls and life force, usually but not always in a manner that produces undead abominations. Sometimes it just rips your soul out of your still living body, or gets used to turn people into fuel for other types of large scale magic, such as a Primal Summoning. Or leeches the vitality of the target, both wounding the foe and healing the user at once, or any number of other things of the same nature.