[renowner]

It already seemed a bit too strong, and about the limit of what i'd feel comfortable with after the first three lines. But then it just keeps going. . .
More damage based on defense? STUN? CLUMSY AND SILENCE AND KNOCKDOWN???? This is insane. Even more so that it isn't status infliction chance.

And it also isn't a finisher. Giving a skill an eight turn cooldown doesn't make it okay either, I feel that it'd be completely insane in that scenario.
Ether invitation is a finisher. It takes 4m and makes them unmarkable for awhile, turning off most of your class.
Execution is a finisher, it only does good damage on low hp opponents, and undoes their knockdown.
The Boxer finisher is a finisher, it turns off your boxer damage multiplier from removing all your Schwartz Sturm.

This. . .This is not a finisher.

The only issues I currently have with the skill are that:

1: It doesn't currently ignore evade. It should. They're already grappled.
2: It knocks back, breaking your grapple. I feel that since grapple doesn't last long already, that it shouldn't knock them away.

The only changes i'd see personally would be then:
3m, 0 cd, only affects a grappled enemy. Status chance to knockdown, 100% chance on a crit.
Can crit. Uses the SWA of your weapon. All as per normal.
But also does not knock them away.

Not much of a change, but still a buff. And it wouldn't fly in the face of grapple's gameplan any longer.

Making it better against monsters is always an option too, so it retains those style points.
It didn't look like the move crit in your first post however, so I can't really compare the damage. It should be doing the same damage as your basic attack, really.

Recall that shaitans get powerful grapples, and that naga get grapples with a certain racial trait, and that Demon Hunter gets a 1m Grapple, (If both are aerial, so you can combo rising tide + Stance Swap + Grapple + Stance swap back + Ozuchi.) and that grapple on its own can be pretty detrimental to some enemy builds. The 6m argument isn't as strong in my opinion here when all these grapple options exist.

I would however also be all for making the ninja cote item itself get buffed to increase your grapple chance a bit.
It'd make sense, and the main drawback of grapple is the fact that its generally a 75% chance if people have that one trait.

Additionally the crit damage is probably just pure guile, I think it should take your weapon into account. And if it doesn't, then the base damage should be raised to 120%.