A THREAD FOR NEWCOMERS!

I generally understand the gist, but I would like someone to explain in more detail, as if to a beginner, the nuances of this technique, in particular: whether the additional undead are raised from one's own or the enemy's elementals, etc., because I don't quite understand it, as I haven't encountered it in practice.

Situation: A computer-controlled hero is present, we launch a surprise attack on him and lose, giving away a lot of artifacts for mana and strength, as well as some magic book (preferably the one in which he is an expert). Then we attack him with a Necromancer, but we don't kill him, but wait until he summons elementals using all his mana, and only then we kill him. After the battle, skeletons (liches) will be raised from each stack of elementals, and there will be a lot of them. That is why this technique is prohibited.