[Snake]

And second strike of the day... It will be more a gimmick, technically, but can Divine Judgement also make the one under its effects unable to use Mercalan domain spells or skills if they have FAI lesser than yours?

It would work like this: Once the invocation is cast, it gains a LV equal to your Scaled FAI, and if the enemy has their Scaled FAI under said LV, they can't use things like Graft, Phoenix, Holy Spark, etc. This, added along with the other debuffs it already does.

Also, the duration should increase by +1 at ranks 3 and 5, to give some reason for Priests to invest in the invocation, skill points wise.

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This Balance Fu is coming mostly from a doubt I had. "How would priests deal with Mercana abuse as Church Knights? The only way to impede someone, as of now, from using heals is by interferencing them with a gun... And that's silly in the theme of 'Holy Crusade' people carrying gunpowder and bullets like outlaws.", so yeah. Adding that will probably cover a little more on their lore too, in my opinion... In regards to Priests be more attuned to Mercala.

Originally I was going to suggest a new invocation being added for them, but I saw that Divine Judgement doesn't really have that much use or ever had at all, so it's good to recycle the idea as an overall buff.

[FiresTFS]

As OP as Priest already is.. This actually sounds like a good idea, since it's an invocation. But this has two sides to it that should be studied.

Strong Point: It's already worth an invocation to disable DEF and RES, which all tanks stack on and replace it with a stat rarely used unless you're strictly a Priest, yourself, making tanks vulnerable all the time. Gaining a secondary effect would be an overkill on that thing.

Weak Point: It is an underused skill and EVEN THOUGH it gives that punishing debuff.. Some tanks come prepared for it with high FAI for the critical evade, too. And what's worse is if their FAI is equal to their DEF and higher than their RES.

Plus, the buff you're intending to give only tends to punish opponent Priests, anyway- so.. It would balance the case and make the skill actually useful if you're going up against a Priest. It's a bit gambling-based on you guessing whether your opponent Priest has higher FAI than you or not, so I think it's pretty cool. I'm not entirely sure about the duration increase, though. Maybe just +1 at R5 and that's it. Going up to 7 Rounds is a bit too much.

tl;dr - +1 to idea, max it at 6 rounds though instead of the 7 you're suggesting.