Gone, Gone the Face of Man!

[Red Fel]

http://i.makeagif.com/media/9-30-2016/_OCD2y.gif

... Rise, the Unchained Summoner!

The prevalence of recent PF UnSum threads has lead me down the primrose path, and into a series of questions.

Question the First: Does UnSum interact with the Synthesist archetype? On the one hand, it seems that it would, given that the Fused Eidolon ability says "n all other cases, this ability functions as the summoner’s normal eidolon ability." Also, the book says, "[f]inally, with the exception of the monk, these classes should work with any of the archetypes from previous books as long as the classes still have the appropriate class features to replace."

On the other hand, the UnEidolon and regular Eidolon follow two very different rules. Also, the Synthesist's Fused Eidolon ability says that it [I]replaces the Eidolon class ability, as opposed to merely modifying it.

Question the Second: Assuming that UnSum and Synthesist can interface, how well do they function together? Do all of the UnEidolon abilities work as normal while Fused, or does doing so horribly nerf the Eidolon?

All of this is to lead up to my next point: Designing a Jason Blood/Etrigan-esque character. That is, a character whose Eidolon takes over (no mechanical difference other than the Eidolon's alignment being dominant) during Fusing. Also, Demon subtype, because obviously.

So, assuming that UnSum and Synthesist play well together, how would you go about building a Synthesist UnSummoner with a Demon Eidolon?

My instinct is to go biped, natural weapons, and just beef it physically (Claws, Imp. Damage, Imp. Nat. Armor, Reach, Large, Magic Attacks, etc.). Is that a bad idea?

[Diarmuid]

I dont see any reason why the UnSum wouldnt work with the Synthesist, and for your fluff I think the easiest thing to do is to use the prebuilt Eidolon Subtypes. There's Daemon, Demon, and Devil that are all built off a Biped chassis (although Daemon and Demon also have quadruped options). They give bonuses on top of those normally gained via Evolution Pool points spent.

[Geddy2112]

Somebody at my table recently just built a similar character, but used it conceptually to create Venom (https://en.wikipedia.org/wiki/Venom_(comics)). Went the same route for claws, bite, super strength hulking brute etc etc. He took the web and climb evolution as well for flavor. Worked fairly well.

The Ueidolon rules work just fine for the syntheist. At least how we played it.

[Waker]

On the other hand, the UnEidolon and regular Eidolon follow two very different rules. Also, the Synthesist's Fused Eidolon ability says that it replaces the Eidolon class ability, as opposed to merely modifying it.
I'd say they work. The passage you mentioned said previous archetypes work, Synthesist replaces Eidolon and the Unchained Summoner has Eidolon, not Eidolon Unchained. How well they work isn't something I can't answer since I haven't really done an in-depth comparison between the normal and unchained Eidolon.
As for making Etrigan, I'd say focus on claws/bite and getting fire immunity. Breath Weapon would also be taken if the Evolution for it wasn't so terrible. Fearsome Presence might be fun. And even if he isn't especially known for it, Weapon Training can definitely come in handy, Blood was a knight after all, he should be able to swing a sword around.
Don't forget to grab Perform, both for your dramatic transformation and for dropping sick rhymes of course.

[Extra Anchovies]

Synthesist uSummoner works fine.

A tip: don't forget about your Summon Monster SLA. Those minute/level summons are a huge part of the Summoner's utility - doubly so with the Synthesist, because you can't send your Eidolon off somewhere else so you and it can perform different tasks.

[Diarmuid]

Have to keep in mind though is that a Synthesist would not be able to use Summon Monster if they are currently using Fused Eidolon.