The Synthesist Archetype of the Summoner from Ultimate Magic is the most complicated class with the most rules exceptions and FAQ explanations of any currently available. The following is a condensed list of all the exceptions and their authoritative sources.

Fused Eidolon A synthesist summons the essence of a powerful outsider to meld with his own being. Instead of appearing as a separate creature next to the summoner, the eidolon appears around the synthesist, so that the synthesist seems to be inside a translucent image of his eidolon. The synthesist directs all of the eidolon’s actions while fused, perceives through its senses, and speaks through its voice, as the two are now one creature.

While fused with his eidolon, the synthesist uses the eidolon’s physical ability scores (Strength, Dexterity, and Constitution), but retains his own mental ability scores (Intelligence, Wisdom, and Charisma). The synthesist gains the eidolon’s hit points as temporary hit points. When these hit points reach 0, the eidolon is sent back to its home plane. The synthesist uses the eidolon’s base attack bonus, and gains the eidolon’s armor and natural armor bonuses and modifiers to ability scores. The synthesist also gains access to the eidolon’s special abilities and the eidolon’s evolutions. The synthesist is still limited to the eidolon’s maximum number of natural attacks. The eidolon has no skills or feats of its own. The eidolon must be at least the same size as the synthesist.

While fused, the synthesist counts as both his original type and as an outsider for any effect related to type, whichever is worse for the synthesist. Spells such as banishment or dismissal work normally on the eidolon, but the synthesist is unaffected. Neither the synthesist nor his eidolon can be targeted separately, as they are fused into one creature. The synthesist and eidolon cannot take separate actions. While fused with his eidolon, the synthesist can use all of his own abilities and gear. In all other cases, this ability functions as the summoner’s normal eidolon ability (for example, the synthesist cannot use his summon monster ability while the eidolon is present). This ability replaces the class’s eidolon ability, bond senses, and life bond.
Ultimate Magic
Fused Link (Su) Starting at 1st level, the synthesist forms a close bond with his eidolon. Whenever the temporary hit points from his eidolon would be reduced to 0, the summoner can sacrifice any number of his own hit points. Each hit point sacrificed this way prevents 1 point of damage done to the eidolon (thus preventing the loss of the summoner’s temporary hit points), preventing the eidolon from being sent back to its home plane. This ability replaces life link.
Ultimate Magic

Changed from a free action to an anytime action by the APG FAQ Sept 9, 2011.
Shielded Meld (Ex) At 4th level, whenever the synthesist is fused with his eidolon, he gains a +2 shield bonus to his Armor Class and a +2 circumstance bonus on his saving throws. This ability replaces shield ally.
Ultimate Magic
Maker’s Jump (Sp) At 6th level, whenever the synthesist is fused with his eidolon, the synthesist can cast dimension door as a spell-like ability using his caster level. This ability only affects the fused synthesist and eidolon. The synthesist can use this ability once per day at 6th level, plus one additional time per day for every six levels beyond 6th. This ability replaces maker’s call and transposition.
Ultimate Magic
Greater Shielded Meld (Ex) At 12th level, whenever the synthesist is fused with his eidolon, he gains a +4 shield bonus to his Armor Class and a +4 circumstance bonus on his saving throws. This ability replaces greater shield ally.
Ultimate Magic
Split Forms (Su) At 16th level, as a swift action, the synthesist and his fused eidolon can split into two creatures: the synthesist and the eidolon. Both have the same evolutions. The synthesist emerges in a square adjacent to the eidolon if possible. All effects and spells currently targeting the fused synthesist-eidolon affect both the synthesist and the eidolon. The synthesist can use this ability for a number of rounds per day equal to his summoner level. He can end this effect at any time as a full-round action. For the duration of this effect, the eidolon functions as a normal eidolon of the summoner’s class level. This ability replaces merge forms.
Ultimate Magic

Questionable Issue
Authoritative Answer
UM FAQ: Summoner: Can a synthesist (page 80) make attacks from his own body (such as manufactured weapons, unarmed strikes, or natural attacks) and attacks from the fused eidolon in the same round?
Yes, but the fused character's natural attacks are still subject to the Maximum Attacks entry in the table for an eidolon of his level. For example, a 1st-level synthesist is limited to 3 natural attacks per round, whether those natural attacks are from the eidolon, the synthesist, or a combination of the two.

If the synthesist wants to use his eidolon's natural attacks and use his own manufactured weapons or natural weapons in the same round, his eidolon needs to have enough limbs to account for all of these attacks. For example, a gnome synthesist fused with a two-armed biped eidolon has two arms it can use to make attacks; if the synthesist wants to make claw attacks with his eidolon's claws and also make weapon attacks (such as with a dagger or staff), he needs to give his eidolon additional arms evolutions to hold those weapons (as an extension of the summoner's own limbs)--a two-armed eidolon can't make two claw attacks and also make a dagger attack or staff attack in the same round.

Remember that the synthesist is still subject to the rules of combining manufactured weapon attacks and natural weapon attacks in the same round (in that the natural weapons are always considered secondary and therefore have a -5 attack penalty).

Remember also that the summoner is wearing the eidolon like a biological, all-encompassing "suit," and the eidolon's shape limits what the summoner can do. If the eidolon doesn't have arms, the summoner can't use his own arms to manipulate objects, make attacks, cast somatic spells, or anything else requiring arms--while fused, the summoner's limbs are trapped within the armless eidolon-suit, and he isn't able to use them to manipulate things. The summoner isn't able to extend his own body parts outside of the eidolon-suit; if he wants to be able to manipulate things with arms, the eidolon needs arms (though tentacles are sufficient for simple tasks).

(Note: It is a matter of flavor and player's preference whether the synthesist floats immobile within the eidolon-suit and its limbs move at his mental command, if the synthesist moves his own arms and the eidolon-suit's arms echo this movement, or if the eidolon-suit is more form-fitting and the flesh-enveloping limbs move in direct response to the synthesist's own movements.)

Note: This clarifies an earlier FAQ error where the summoner's weapon attacks counted toward the number of attacks on the table. (and the summoner's arms could stick outside of the eidolon suit.)

—Sean K Reynolds, 08/02/11
UM FAQ: Summoner: Can a synthesist (page 80) benefit from the Skilled evolution even though his eidolon doesn't have skills of its own? What about the Ability Score Increase evolution applied to a mental ability score?
If the synthesist's eidolon has the Skilled evolution, the synthesist gains the bonus to that skill.

If the synthesist applies the Increased Ability Score evolution to the eidolon's mental ability score, it has no effect on the synthesist because he uses his own mental ability scores.

—Sean K Reynolds, 8/02/11
UM FAQ: Summoner: Can a synthesist (page 80) use spells on himself that don't affect outsiders, even though he is treated as his own type or the outsider type, "whichever is worse"?
Yes, because the normal eidolon's share spells ability says "A summoner may cast spells on his eidolon even if the spells normally do not affect creatures of the eidolon's type (outsider)," and the synthesist archetype doesn't change that.

Note: This is a revision of an earlier question that used enlarge person as an example. FAQ pending about size-changing magic and a synthesist (because of the limitation of "the eidolon must be at least the same size as the synthesist" rule). (So far, they haven't added the size addendum.)

—Sean K Reynolds, 08/02/11
UM FAQ: Summoner: Can a synthesist (page 80) use the aspect ability to apply the improved natural armor evolution to himself? Will this stack with the improved natural armor of the eidolon-suit?
A synthesist can use aspect to increase his own natural armor. However, when fused, he gains the eidolon's natural armor bonus, so this doesn't stack with his own natural armor bonus.

(Using the ability in this way is still useful for times when the eidolon is banished, dead, or dismissed.)

—Sean K Reynolds, 08/10/11
UM FAQ: Summoner: Does a synthesist (page 80) keep any armor bonuses or other benefits from his armor when he is fused with his eidolon?
No. The eidolon is, in effect, the synthesist's armor, and any armor the synthesist wears does not function while the eidolon is present. Fortunately, this doesn't mean the synthesist has to remove his armor when wearing the eidolon, and when the eidolon is banished/dismissed/killed, the summoner's worn armor immediately begins functioning again.
—Sean K Reynolds, 08/02/11
UM FAQ: Summoner: Does a synthesist (page 80) use his own Hit Dice or his eidolon's Hit Dice for evolutions based on Hit Dice, such as Breath Weapon and Web? What about his BAB for making attacks?
The synthesist uses the eidolon's Hit Dice for the effects of evolutions, not his summoner Hit Dice or his total Hit Dice.

When fused, use the eidolon's BAB instead of the summoner's class BAB, and add in BAB from other sources as normal. For example, a fighter 19/summoner 1 normally has a total BAB of +19 (+19 from fighter, +0 from summoner), and when fused with his eidolon this increases to +20 (+19 from fighter, +1 from the 1st-level eidolon).

—Sean K Reynolds, 08/02/11
UM FAQ: Summoner: How does a synthesist (page 80) heal damage to his eidolon?
Because the eidolon gives the synthesist temporary hit points rather than having a separate pool of normal hit points, effects that cure hit points don't restore the eidolon's temporary hit points. This technically leaves you unable to heal the eidolon. To remedy this, effects that specifically restore hp to an eidolon (such as rejuvenate eidolon) restore temporary hit points to a synthesist's eidolon. This does mean those spells end up as a sort of must-have "spell tax" for synthesists, but the advantage of being a synthesist is your eidolon's hp are a buffer between you and damage, unlike a normal summoner who can be targeted separately from his eidolon.

Even the Fast Healing evolution, or other fast healing or regeneration effects, restore hit points rather than temporary hit points, so they heal the summoner, not the eidolon's temporary hit points.
—Sean K Reynolds, 08/02/11

UM FAQ: Summoner: If a synthesist (page 80) takes the swallow whole evolution, does he use his own hp or the eidolon's temporary hp to determine the hp of the "stomach"?
Use the summoner's hp to determine this. Obviously the summoner isn't actually swallowing the target, so it's merely contained within the eidolon-suit, but the hp of the "stomach" is based on actual hp, not temporary hp, so it's simpler to use the summoner's hp rather than create a specific exception to have this based on the eidolon-suit's temporary hp. Fortunately, this means effects that boost a summoner's hp (such as bear's endurance) increase the hp of the "stomach."
—Sean K Reynolds, 08/10/11
UM FAQ: Summoner: What happens when a synthesist (page 80) takes ability damage or drain while the eidolon is present?
The eidolon takes Str, Dex, and Con damage or drain, the summoner takes Int, Wis, or Cha damage or drain. If this bring's the eidolon's Str or Dex to 0, the fused summoner/eidolon is helpless (though he can still dismiss the eidolon). If this brings the eidolon's Con to 0, it dies and returns to its home plane. If the eidolon goes away (whether dismissed, banished, or killed), the summoner immediately begins using his own Str, Dex, and Con.

There is no "spillover" for extra ability damage or drain beyond what it takes to reduce the eidolon to 0; if an eidolon with Constitution 1 takes 3 points of Con damage and dies, the summoner doesn't take the "extra" 2 points of Con damage. However, ongoing effects (like continuing poison damage) would affect the (non-fused) summoner after the eidolon disappeared.

Remember also that the eidolon does not heal naturally. Just like hit point damage, any ability damage or drain the eidolon has persists between when it is banished/dismissed/killed and when it is resummoned. The exception is an eidolon killed by Con damage (which, if resummoned, would still be at 0 Con and immediately die and vanish); in this situation, the resummoned eidolon is cured of 1 point of ability damage or drain so the amount of damage or drain is 1 less than its actual Constitution score, allowing it to live.

Effects that cure ability damage or drain work on the eidolon's ability scores when it is summoned.

—Sean K Reynolds, 08/02/11
UM FAQ: Summoner: What happens when a synthesist (page 80) takes a penalty, suffers an affliction, or has an ongoing effect when the eidolon disappears?
These effects persist on the summoner after the eidolon is gone. For example, a ray of enfeeblement on the fused character continues to affect the summoner after the eidolon is gone, as would a bestow curse spell or acid arrow spell, as would continuing poison damage. These effects apply to the character as a whole, and just because the eidolon is gone doesn't mean the summoner is freed from the effect (ability damage is tracked separately, as described in another FAQ, because it is a separate game statistic that the summoner "borrows" from the eidolon). If the condition or effect ends (whether from its duration running out, being cured/negated/dispelled, and so on) while the eidolon is gone, it doesn't return when the eidolon is resummoned. This applies to beneficial effects as well as harmful ones--an invisible summoner isn't suddenly visible if his eidolon disappears, nor does he become invisible again if the duration ran out while the eidolon was gone.
—Sean K Reynolds, 08/02/11

Paizo Forum Question: Haven't found that issue with search. Synthesist being Humanoid doesn't have any proficiencies with natural weapons, correct? Common sense dictates that the ruling will be that yes, they're proficient.
A creature is always proficient with its natural weapons, even if it gains those natural weapons from something other than its original race: barbarian rage power, magic spell, magic item, class ability, or whatever. So a synthesist using his eidolon-suit's natural weapons is proficient with them.
—Sean K Reynolds, Wed, Aug 10, 2011
Paizo Forum Question: Can you use the fused physical stats to qualify for feats? Or must the synthesist use his own stats?
Just as a Str 11 character wearing a belt of strength +2 bumps him to Str 13 and allows him to take Power Attack, you can do this. You just couldn't use the feat without the belt/eidolon-suit.
Sean K Reynolds , Wed, Aug 10, 2011
Paizo Forum Question: What happens when a synthesist gains inherent bonuses to physical stats before he has split form? Is his eidolon suit's stats increased or his own or both?
If you apply the inherent bonus to the summoner's physical ability score, that ability score is replaced by the eidolon's ability score. So if you want to apply an inherent bonus to your eidolon-suit's physical ability score, use the item/spell/etc. while fused so it applies the bonus to the eidolon's ability score.
Sean K Reynolds, Wed, Aug 10, 2011,
Paizo Forum Question: However, that same phrase that seems to be the only way to justify letting Skilled work for the synthesist seems to also indicate that the synthesist would get her own copy of Increased Ability Score (Intelligence) if the eidolon took that evolution (or would get to "use" the eidolon's copy i.e. gain the benefit from it herself as well).
It's basically a ruling to prevent players from cheesing out an extra +2 to the caster's casting ability score by selecting the evolution. Them's the breaks.
—Sean K Reynolds, Wed, Aug 10, 2011
Paizo Forum Question: In the case of the synthesist, wouldn't Enlarge Person affect both since the summoner cannot be targeted separately from his eidolon. So the relative size of the eidolon and the summoner remains the same.
That is how *I* think it should work, yes.
—Sean K Reynolds, Wed, Aug 10, 2011
Paizo Forum Question: Was asked differently, but the real question was "Can the eidolon wear bracers of armor?"
Bracers of armor are not armor. They don't take up the "armor" body slot.
—Sean K Reynolds, Sept 13, 2011

Feat Qualification: Does the fused eidolon qualify as an "eidolon class ability" for feat qualifications?
Yes. Since the Ultimate Magic and Ultimate Combat books do not contain a feat that requires a "fused eidolon", the fused eidolon qualifies as a normal eidolon for feat selection.
—LPF Judge Vote Aug 28, 2011
      Then later, a confirmation:
From the archetype:
"In all other cases, this ability functions as the summoner's normal eidolon ability..."
Sean K Reynolds, Nov 11, 2011
How often can the Extra Evolution feat be taken?
Ultimate Magic wrote:
You can gain Extra Evolution multiple times, but only once for every five summoner levels you possess.
Oni_NZ wrote:
The wording is a bit ambiguous, at what levels can it be taken?

It should be like the correction to the Improved Natural Armor feat: once at 1st, then again at 5th, 10th, 15th, and 20th.
Sean K Reynolds, Nov 9, 2011

So to summarize: The class pretty much requires the player to keep two sets of stats in their wiki page, the normal caster and the fusion. Hit points are tracked separately for the caster and the eidolon-suit. All damage is to the eidolon-suit first. All healing is to the caster, unless it is from the Rejuvenate Eidolon or Life Conduit spell series. Hit points can be transferred with the Fused Link (as an immediate action) when the attack would have put the eidolon at or below zero, and only for the amount that is sustained in that attack. Other damage repair spells like Lesser Restoration work normally to repair their type of damage (non-HP). At zero HP, the eidolon suit disappears unless the Fused Link was used to keep it around. If the suit disappears because of damage down to zero HP, it cannot be recalled by ritual for 24 hours. The second level spell 'Summon Eidolon' can recall the eidolon at 1/2 its normal HP for a limited time.

The suit has to have at least one set of arms for the caster to cast somatic spells. Physical armor does not help the fusion, but does not need to be removed by the caster before summoning the fusion. It is simply absorbed and ignored like a druid's wild shape without the wild armor property. The fusion is proficient with its natural weapons and simple weapons. Multiclassed BAB is a summation like any other class, not a replacement.

The Skilled evolution is usable by the caster while fused. The ability increase evolution to mental stats would be applied to the eidolon (which still can think and talk) and not the caster's stats. So, level based inherent stat bumps should be taken on mental stats by the caster, and physical stats by the eidolon to be most efficient. Items worn that produce stat bumps are temporary bonuses, so they apply to both sides at the same time. The Enlarge Self spell works.

The Synthesist qualifies for Extra Evolution, Resilient Eidolon, Summoner's Call and Vigilant Eidolon. It does not qualify for Defending Eidolon, Focused Eidolon or Sense Link. The Improved Share Spells is pointless, as it is a default state for the Synthesist.

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