Additionally two of the required feats add very little mechanically, though they do have cross-purpose in filling requirements for a few other prestige classes.
All that being true, the class itself is only desirable up to a certain point. Class Level 5 for increased critical multiplier, or Class Level 7 for increased critical range. Further Class Levels beyond 7th level are essentially wasted, and better spent in other classes.
For this very reason, I find it appropriate to introduce a Capstone Feat to the Weapon Master class. Something in line with the class fantasy, how it operates, and what its strengths are.
DISCLAIMER! — All numbers presented here-after are for proof-of-concept only, and not meant to represent how the class feature should be in its final form. Adjustment and balancing is to be done by the developers, if the suggestion is approved.
Ki Ruin
Effectively this gives the Weapon Master class some additional usage of the Ki Damage feat, which itself is, considering the bugs with damage calculation for Ki Damage, quite lacklustre.A 10th level Weapon Master can expend 2 of their daily uses of Ki Damage to infuse their Weapon of Choice with the Ruin property. This infusion of lasts 5 rounds plus 1 round per 3 ranks in Craft Weapon, capped at 20 rounds (45 ranks in Craft Weapon).
Upon activating this ability the Weapon Master must choose an enemy type for which to apply Ruin to their weapon. Once a weapon has been infused it cannot be re-infused until the current infusion wears off.
Ruin allows for critical hits against the chosen creature type, even if that creature is normally immune to critical hits, or an effect is making them immune.
Similar to the Dervish Dance ability, Ki Ruin scales duration with investment into a Skill. In this case Craft Weapon. At 45 total ranks in Craft Weapon the Ki Ruin feat will last 2 minutes (20 rounds), for a total duration of 10 minutes at 5 uses per day. Unlike Dervish Dance, however, which scales through Perform and thus cross-purposes for Bard, there is no cross-purpose for investment into the Craft Weapon skill.
Edit: Renamed the ability to Ki Ruin to properly convey what it is supposed to do. Mistakenly named Ki Bane because I mixed up the names. Thank you to Kitunenotsume for pointing this out.