To start: thnx for the work!I have some stuff, input, questions, and suggestions.
It's good that the binder got a handbook in a more structured format; but the old version by jameswilliamogle had a lot of information that was useful, on combo's, bloodline's, items, etc. I can imagine you won't include all of it, but wouldn't it be a good idea to port the entire version over to here as well? Would be a shame to see that info got lost if BG ever gets deleted.
Since you mention infinite out of combat healing pretty early in the handbook, it's worth mentioning also the feat healing devotion: for one turn undead attempt, you can give somebody fast healing. Combine with tenebrous, and you have another option for ... and yeah, it's the same abuse as is already mentioned in this discussion, but hell, it's RAW, and plenty of party's play at an optimization level where it's not that weird.
I noticed that you don't mention the semi-legal vestige 'green lady', from the path of worms adventure path, iirc. The link to it is: http://paizo.com/paizo/messageboards/paizoPublishing/olderProducts/dungeon/generalDiscussion/archives/statsForTyralandiScrimm&page=1#9
Of course, it's not official wotc, but then again, when age of worms started, I think it was, so it's as close as legal as it gets.
If you mention it, it opens up a lot more possiblities for turn undead abuse; binding both the green lady and tenebrous gives 2 turn attempts ever 5 round, giving access to unlimited use of devotion feats that cost 2 turn attempts, possiblity for DMM-abuse for an anima
cleric, etc. etc.
With a divine version of the anima cleric, also divine feats can be abused by tenebrous, I think especially about divine vigor (CW) and sacred healing (CD, hello again infinite out of combat healing)
As a race, I think it might be worth it to include dragonborn. The stats are ok for a binder (+2 con -2 dex), but even more important:
- the flying version is a way for a binder to gain permanent flight, what can't be done by it's class features
- the version with breath weapons synergizes really well with a version of the binder that focusses on breath weapons. Iirc, abilities given by vestiges don't work as requirement for feats; so normally, breath weapon feats are out of the question, but as a dragonborn, it's possible to get max breath, entangling exhalation, etc. etc., and it also gives a breath weapon, making it at low level possible to use two maximized entangling breaths in the first two rounds of combat
Finally, I'm wondering about the KotSS, wether it's as mandatory as the handbook suggests. Of course, it's good. But comparing binder 20 with binder 15 / KotSS, the differece is that binder 20 has:
2 extra feats (bonus feat and didn't spend weapon focus on a weapon you won't use after gaining entry to KotSS anyway)
2 more pact augmentations
no med and heavy armor proficiency, no shield proficiency and no martial weapon proficiency
no vestige power
no vestige protection (aura)
no vestige surge
Weighing all that, against each other, the KotSS seems a little more attractive (barring abuse of outsider status, then KotSS is of course much more powerful). But not that much... and a major disadvantage of the KotSS is that it ties the character to a specific vestige... that comes at the cost of flexibility, which is a major point of playing a binder in the first place. So I understand why i's recommended as a strong option, but a 'must have'.