A Guide to the Synthesist Summoner (Pathfinder)
At one point during the playtest rounds for the Advanced Player’s Guide, a Paizo developer referred to the summoner as the class for people that wanted to “play the monster.” The synthesist summoner archetype (Ultimate Magic 80) takes that idea to eleven, letting the summoner become his eidolon from the get-go at 1st level (normal summoners can do this at higher levels, though the mechanics work somewhat differently). This has the added benefit of making the summoner slightly less complicated, in general, to play at the table (since the synthesis doesn’t have to decide her own action and her eidolon’s action every round).
The majority of the synthesist’s class features function just like the regular summoner’s, with most of the changes being made simply to account for the fact that the synthesist and her eidolon are one creature, rather than two. When fused with her eidolon, the synthesist gains the eidolon’s Strength, Dexterity, and Constitution scores (meaning that, generally, she doesn’t need high scores in these of her own), while keeping her own Intelligence, Wisdom, and Charisma scores. She adds the eidolon’s hit points to her own as temporary hit points, and, when those temporary hit points hit zero, her eidolon heads back to its home plane. She also gains all of the eidolon’s evolutions and any special abilities (like darkvision and, if she’s high enough level, multiattack). A synthesist’s eidolon doesn’t gain its own feats and skill points, though, so the synthesist is going to want to make sure that her feat selection allows her to perform well when she’s fused with her eidolon.
The synthesist gains fused link instead of life link at 1st level, allowing her to expend her hit points to prevent damage to the temporary hit points she gains from her eidolon, allowing her to keep it around longer. As with most of the synthesist’s abilities, this has the same essential function as the original summoner’s ability—it’s just reworked slightly to account for the archetype’s special nature.
Shielded meld replaces shield ally at 4th level, granting the synthesist a +2 shield bonus to armor class and a +2 bonus to saving throws whenever she’s fused with her eidolon. At 12th level, these bonuses improve to +4 via greater shieleded meld. Maker’s jump replaces maker’s call and transposition, allowing the synthesist to use dimension door as a spell-like ability once per day for every six levels she attains. Finally, at 16th level, the synthesist gains the ability to split forms with her eidolon, instead of the normal summoner’s ability to merge forms with her eidolon.
The abilities in general don’t need a lot of analysis, since they’re not vastly different from the standard summoner’s. Greater care, though, needs to be taken when choosing feats, as being a synthesist changes the value of a lot of feats for you. Your eidolon gains no feats of its own, so when you fuse with it, it’s relying on your feats to succeed in combat. Unlike a standard summoner, you’re probably going to want melee-centric feats like Power Attack. The feats you choose should complement the evolutions that your eidolon has. You’re not going to want Power Attack if your eidolon focuses on using a longbow—you’ll want Deadly Aim instead.
Try to focus on feats that are effective on their own, as you don’t get the same sort of bonus feats that a fighter does, and you’ll also probably want to spend at least some of your feats on caster-related things, since you’ve still got six levels of spells to work with as well as your summon monster class feature. You can still use the spells while fused with your eidolon (though, just like a normal summoner, your summon monster ability is off limits when you’re eidolon’s around).
With those ideas in mind, here’s a basic sample build for a synthesist. The summoner class as a whole is hard to pin down to one particular build style, since you can do so many different things with your eidolon. This build is simple and straight-forward, but it will perform well as a secondary melee-combatant. Remember when building your own synthesist that you don’t need high physical ability scores since you get your eidolon’s scores in those when you fuse, but you do need to make sure you meet any requirements for feats that you take (such as the Strength of 13 this build needs to take Power Attack).
Human Summoner (Synthesist 5)
Ability Scores (15 Point Buy): Str 13, Dex 8, Con 12, Int 10, Wis 12, Cha 17
Fused Ability Scores: Str 20, Dex 14, Con 14, Int 10, Wis 12, Cha 17
1 Improved Initiative, Extend Spell
3 Power Attack
5 Weapon Focus (claw)
Eidolon (8 Points): Base Form biped; Evolutions ability score increase (Str +2), limbs (extra arms), claws (on extra arms), improved damage (claws), grab
I’m always a fan of Improved Initiative for any spellcaster, as being able to get buffs up early or give your allies the advantage with a well-placed battlefield control spell can really stack the odds in your party’s favor. Power Attack makes sure that your four claw attacks hit harder when they do hit, allowing you a good chance of getting a grab off on any pesky foes that might be giving your group trouble (which gives you even more options for battlefield control—never a bad thing!). Extend Spell can serve a lot of different purposes, but it’s on this build to help you keep up useful buffs like enlarge person and (later) bull’s strength as you explore whatever nefarious dungeon your Game Master has you delving into. This build takes it at level 1, before it's highly useful (largely because the summoner's low base attack means you can't take Weapon Focus or Power Attack at level 1). You can easily substitute it for another feat (like Spell Focus (conjuration) to unlock Augment Summoning).
As always, this build’s only meant as an example. The summoner is one of the most customizable classes in Pathfinder, so it wouldn’t do to constrain yourself to some build you found on the internet.
That’s it for this brief guide on the synthesist!