Iaijutsu masters are not trained as assassins. Most of them are courtiers, diplomats, artists, maybe spies. While they got Ki training, which enhances their martial abilities (including the ability to move silently), and they are educated in medicine (and hence, the use of poison), they have no training in the art of assassination, in killing silently, in hitting vital spots.
Instead, they master the art of Iai, the technique of drawing a weapon from its sheath at blinding speeds and striking with incredible power.
Iaijutsu is an art form, sometimes used for duels when matters of honor are at stake. It is not a tool of war, and few Iaijutsu masters consider themselves warriors. Duelists, possibly, but not men of war. An art with such strict rules and limits is ill suited for use on a battlefield. A single devastating strike is enough to win fights against single opponents, but against multiple opponents, an Iaijutsu Master will have to rely on finesse, trickery, and his allies.
Alternate class features
Iaijutsu Strike:
An Iaijutsu master can perform a devastating iaijutsu strike, a technique where where he increases the power of his blow by drawing his weapon from the sheath and striking in the same motion.
The iaijutsu masters first attack deals additional damage after drawing the weapon that the attack is made with from the sheath. The attack has to be the action following drawing the weapon, and in the same turn. The weapon has to be a light or one handed slashing weapon.
Note: this is not precision damage. It is added to the base damage of the weapon, and multiplies on a critical hit, or with feats like Vital Strike.
If the Iaijutsu Master has a base attack bonus of +1 or higher, he may sheath a weapon as a free action combined with a regular move. If you have the Two-Weapon Fighting feat, you can sheath two light or one-handed weapons in the time it would normally take you to sheath one.
Any feats, Ninja Tricks, Rogue Talents and other abilities that modify the effects (but not the requirements of performing it) of Sneak attack also apply to Iaijutsu strike. If you'd gain Sneak attack from another class, you may instead add its damage to Iaijutsu strike. You have to make this decision when you get the Sneak Attack progression from that class (or when you take your first Iaijutsu master level) and you can not modify your choice later.
So, I forgot to mention, but if it wasn't obvious, Iaijutsu strike replaces, and has the same advancement as sneak attack.
I also had one more ninja trick idea I forgot to add.
Force Slash:
A ninja with this ability may spend a ki point when making a melee attack to treat that attack as if his reach was increased by 30 feet.
For every time the ninja selected the Deadly Range trick, his reach is increased by an extra 10 feet when using Force Slash.
Force Slash Flurry:
A ninja with this ability may spend a ki point as a swift action to affect all the melee attacks he makes until the end of his next turn with Force Slash.
The ninja must have the Force Slash trick before selecting this trick.
I'm pretty sure there is already a samurai archetype that has an iajutsu strike, seeing as its an art form meant to settle disputes of HONOR. Ninja's weren't really known for their honor
See Iaijutsu Focus from 3rd edition. It was a skill that could be used immediately after drawing a weapon. The higher your skill check, the more damage dice you used on the first attack. Though, if you have iterative attacks and don't draw a weapon between attacks, you don't get the bonus damage multiple times per round. It was supposed to simulate a Kenshin style of fighting. The skilled warrior would draw his blade and cut his enemy in the same motion. In reality, a combination of Quick Draw and some free action sheathing of weapons made this skill better than Sneak Attack... And the Rogue would dip one level of something that got it as a Class Skill... ALL THE D6s.
Don't mix up Samurai the class and Samurai the social class/job. An Iaijutsu Master may be a Samurai (social class) and a Samurai (class) may be a Ninja (job). Plus, this is an Iaijustu Master, not a Ninja.
I know of iaijutsu focus and it was a very great skill (although way more difficult to pull off than an SA, it scaled way faster and stacked; when you could SA, you could Iaijutsu strike), and that's exactly why it's better as a class ability instead of a feat that everyone with a spare skill point would love to get.
