Discussion of the Necropolis faction.

In other words, if your army consists only of basic units, will you always have Necromancy I until you get elite units or something even stronger? And can you then, for example, use Necromancy III to resurrect basic units? It seems like some kind of nonsense.
And also, for Blood magic level 3, how many turns does it block, or only once during the entire battle?

No, the level of necromancy does not depend on what you have in your army. You will have Necromancy I until level 5, and then access to level 2 will become available.
For example, until level 5, it doesn't matter how many points you accumulate; you will still only be able to resurrect Core units and for a not very large number of hit points. After the hero reaches level 5, he will be given Necromancy II. In battle, when you accumulate a sufficient number of special points (I don't know what they are called, approximately a little more than a quarter of the circle), you will be able to resurrect any of your units, up to elite creatures + a larger number of hit points, and so on, increasing with each level.