[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.