Birthright: Chimaeron

A better approach might be a regency loss/immediate loyalty drop from such an action.

I'm not sure what examples there's been of regency loss for alignment infractions in this game, but a lawful character could lose regency from destroying their peoples' traditions, a chaotic one from restricting their personal freedoms, an evil one from making themselves look bad, or a good one just for inflicting harm on them. Aside from the act itself, the nature of the temple could make it harder or easier to justify actions against it. If the regent's alignment is similar to the temple's, then it seems they'd be in more trouble, regardless of why they're fighting it.

The loyalty drop is already built into occupation, though if it drops further when a temple is destroyed, that'd result in an automatic rebellion. That might be a bit much, since the temple can already agitate to rebellion easily when a province is occupied.

Defection is too good.

Inclined to decrease to:

1 Tier +1 per 2 levels (rounded down).

So 1 at level 1, 2 at level 2, 3 at level 4, 4 at level 6 etc.

Well, in an early game, that is devastating. The invasion of 15 units, if u can get 3 units defected, it makes a serious impact. For a not so early game, where you have most of units trained at Tier 3, a level 9 wizard/priest can defect 3 inf, for example. The battles we got earlier were of around 60 units. I doubt 3 units will make a serious impact. Early?. Totally agree its a pain, but in advanced games, its a mere nuissance relatively. Its just where you stab what it matters. If a warlord spreads all his units to cover most area, logical a well placed defection can hurt. But with monolythic armies of 30 units?. No way..,)

We are seeing devistating effects from defection in the current game. The cost you pay for defection is basicaly the cost it takes to train the unit. As they fight the remaining units you get a 2-1.
In a late game the effect wont be as devestating but it still is the most cost effective dmg a wizard can do. What other spell would you rather cast to do dmg on an army? I thing the suggested change by Juan will help a bit but still the spell will be used whenever possible.

I had thought there was some type of a save... But that was not the case. Maybe add a save vs the spell?

Let's neuter wizards more....

The spell is fine I think. Yes it can suck to have levies turned en masse. Ensure you have magic defenses in place and are opposing enemy casters.

Target enemy casters with assassination. It should be a scary thing to know a wizard can turn a battle/war upside down with a well-placsd spell.

Maybe for a certain price you can "stun" units so that they are unusable and for a more expensive price you can defect them.

Or change it to a confusion type spell where they defect or attack as normal or are stunned or wander off etc. That way it's unpredictable but still fun and you dont have to raise the price so high that people get sick of playing mages.