The Lores I mentioned work the same as any other magic- Winds, High, Dark, Great Maw, Big and Little Waaagh, etc.
My understanding is, you try to cast a spell, you roll dice (d6s) equal to your Wizard level, if you get any doubles that's a miscast, if you get over the cost of the spell you successfully cast it.
Warrior Priests on tabletop (basically all Sigmar, except for Ulric back in Storm of Chaos) have a set of Battle Prayers that generally have somewhat smaller effects and much lower range (only effects the unit the Warrior Priest is part of) but I don't think they miscast. Not sure you roll for them, either.
If I got anything wrong, anybody with actual tabletop experience please correct me.
So I think you're basing this on some slightly older mechanics. Magic changed between editions but as of 8th it worked like this:
1. Roll 2d6 for the Winds of Magic. The total is your power dice (how many dice you get for casting spells). The higher of the two dice is used as your opponent's dispell dice (how many dice they get to stop you from casting spells).
2. Channel more dice (every Wizard and some other models roll a d6, 6s add 1 die to the owners respective pools of dice). Also add dice from anything that says you get more dice.
3. Pick a spell one of your Wizards can cast. Roll a number of dice from 1-6, then add the Wizard's casting level.
4. If you rolled high enough to meet the casting value, the spell is cast. If the total on the dice you rolled is 3 or less, the spell fails even if you meet the casting value. If you failed to meet the casting value, the casting. Wizard cannot attempt any more spells this phase. If you rolled 2 6s, you have cast with irresistable force and also you have miscast, skip the next step. Rolling 2 6s means your spell succeeds, regardless of if your meet the casting value.
5. Your opponent may choose to dispel. They pick a wizard to dispel with (if they have no wizard they still get to try to dispel, you just don't add a Wizard level to your roll). They roll as many dispel dice as they desire, and add their dispellign Wizard's level to the result. If they equal or beat your casting roll, the spell is dispelled. If They roll double 6s, the spell is dispelled regardless of whetehr they beat your roll.
6. Resolve the results of your spell (damage etc.). If you miscast, roll on the miscast table.
Priest prayers are a particular form of spell known as an innate bound spell. Bound spells do not get Wizard levels added to their casting rolls, and if they miscast, all that happens is the item breaks. If innate bound spells miscast, the model simply cannot cast anymore this turn. Bound spells also typically have very low casting values.
He can't just go, "Mathilde, you're so special, you get to do bad magic". I imagine anything active would be quite dicey. Building, not just studying a waystone that deliberately creates dhar would be such a thing.
On the other hand, the Emperor could, in principle, allow someone to freely do bad magic. In most cases, it would just require a huge amount of political capital, and face pushback from just about anyone. But if we ever get that far, we might have to revisit that topic.
I think the Emperor would have to pass a law to allow a Wizard to freely use dark magic. Laws and ideals > the Emperor in the Articles.
There's probably something in Ghrond...
Ah yes, a perfect target for the Library to book share with.