Iaido is kind of a really awful martial art as a whole - it's almost sole focus is on drawing the sword, which is something that will only matter in a vanishingly small number of fights you are likely (historically or in game) to get into. There's a reason why it is the only martial art of its kind, while others usually have a small element of drawing quickly in them (Fiore, possibly I.33, some rapier treatises), if that.

That means you will have to bend over backwards to create a class centered around it, and it will probably be very clunky, or not very... iaido-like. Something like this, a specific skill inside a martial art, is more suited to a feat.

As for what that feat should be, quick draws are useful in only a handful of areas.

First area is weapon transitions on the battlefield, from yari or arquebus to a sword.

Second area is defending against ambushes that have for whatever reason (stupidity, legal restrictions, terrain) decided not to use guns or bows to ambush you. So, not a lot of military ambushes, but possibly some self defense urban ones.

Third area people bring up, one that is really, really dumb, is assassination - in this case, the problem isn't attacking soeone, it's getting away with it, and for that, melee assassinations are kinda useless, so you will likely not use this and/or not need to defend against it. Japan had few notably fanaticised groups willing to do it, and that is all I can say under forum rules.

Focusing on first and second areas of use, then. Iaido doesn't really gain you initiative in the fencing sense, because it is almost invariably a reaction to something. It is both offensive and defensive in nature, and it only has an effect shortly after you draw your sword, after that, it's just a normal sword fight. It has absolutely nothing to do with any mysticism or wisdom, it's just a way to kill people more good.

With that in mind:

Iaijutsu training

Prerequisites: initiative bonus of at least +1

If you start a round with your sword drawn and in a scabbard and draw it, you can add your initiative bonus to your AC, attack and damage for one round.



Feel free to tweak it to work better with particular DnD version action economy (reaction, free or swift action to draw etc), and to double or triple the bonus to tweak its power. It only lasts for one round, after all.