The book's first outsider! Will it be worth it, despite the hefty 16 RHD?
Phantasmal slayers are medium-sized (okay-ish), with the Evil, Native, and Incorporeal (!) subtypes. They have a decent 60 ft. fly speed, and considerable bonuses to the five stats they don't lack. The 26 charisma is especially useful.
The slayers have a number of interesting abilities, such as immunity to Fear, 100 ft. telepathy, SR 22, and of course the huge batch of goodies that comes with being incorporeal. More notable, however, are their special qualities.
The first of these is Penetrating Wound, which causes the slayers' 2d6 touch attacks to penetrate all DR that doesn't require a specific material (which at this level will probably let you break through most DR). However, this ability does not matter a whole lot.
Phantasmal Facade, on the other hand, does. Basically, every creature that sees the slayer for the first time in 24 hours has to make a save. If it succeeds, it's dazed for a round. If it fails, it has to make an additional saving throws versus death the next time the slayer touches it, which inflicts minor damage and allows a new save to disbelieve should they succeed that one. Oh, and fear immunity doesn't apply here.
In other words, anything that looks at a slayer (there's no range limit either), gets caught in a nasty cascade of saving throws that either ends with death or dazedness (not something you want to get hit with when fighting high-level parties). Something with bad Will saves and good Fortitude saves might be able to delay either effect for a few rounds, but CR 15+ monsters with bad Will are screwed anyway for a bunch of different reasons.
In addition to this, slayers get a bunch of SLAs that are mostly bad, with the notable exception of 1/day Feeblemind, which is decent enough to spend an action on. Y'know, sometimes.
However, this monster is so crippled by immunity to mind-affecting effects (which most creatures at this level will have) that I see myself forced to assign -0 LA here.