What is the spell Steal Spirit supposed to do?

it can only give you the apprentice level for the selected school. for example, if your champ has water 2 and your sov doesn't have water yet, you can steal water apprentice (water 1) from the champ.

At one stage, you could sacrifice a hero with say, fire 2, and if your sov has fire 1, you could go up to fire 2. I think.

Seems like a more useful spell now that there are so many extra heroes, though. Remember to sell/trade all their equipment first.

You can only get new spelltraits that you don't have, and that includes levels. So, if you're 2 water and kill someone with 3, you will get it; but not if the hero has 1 water. 

All in all, I like it. It is a good way of getting rid of excessive heroes...

@Bigorra Are you sure about that? I haven't used the spell in ages but last time I tried it was only capable of giving lvl 1 or magic schools, not increasing the lvl of existing schools. I might use it again if it can level up my magic schools.

Yes, it's always been able to increase the levels of schools, even ones you can't normally learn.

not working in my game, it just kill hero with items lost in process if not traded pefore "operation".

level 1 or nothing

this makes any hero without spells or with spells you already have a bit useless. 

You can only steal one spell level at a time. E.g., if the champion has Fire 2, and you have no Fire magic, you get Fire 1. If you already have Fire 1, you can get Fire 2 from this champion.