Having not played PF, I am puzzling over the eidolon, and arrive at the conclusion that it is a super-familiar / super-homunculus that is called and created by a spell caster(summoner?). Is that about right.
@KorvinStarmast Lore-wise, the summoner forms a bond with the eidolon (an outsider from another plane), and it gets stronger as the summoner gets stronger
Mechanically it's conceptually similar to an animal companion, plus it gets customizable evolutions every level (extra arms, wings, different senses and defenses and immunities, etc)
Balance-wise they are gross (because of how natural attacks function in PF) and they had to revise the class, so now there's the Summoner (from Advanced Players Guide) and the Unchained Summoner
@KorvinStarmast Since there are so many options and ways to build eidolons, it can result in some weird rules interactions, so it's understandable that they raise many many questions
Unchained summoner is a complete debuff of the class to make it more balanced
APG summoners would get spells at strange levels, like getting Haste and Stoneskin before wizards do
So unchained fixed that to match that of a typical 2/3 progression caster
Unchained eidolons are themed (angels, devils, elementals, djinn, robots, etc.) and come with a package of benefits (that apply at different levels), so they have fewer customizable evolutions and thus less wacky optimization
@KorvinStarmast tl;dr Eidolons were OP martials relative to the other martial classes
Oh also Eidolons can't die, usually, which is why they were so often optimized for damage output
@KorvinStarmast As casters Summoners are meh. They're like conjuration/transmutation sorcerers with fewer spells.
At lower levels, when the other classes struggle with Summon Monster spells, Summoners are absurd because, under certain conditions, they can 1. summon as a standard action and 2. the summons last 10x longer
It doesn't work if the eidolon is out, or other fast-summoned creatures are out. But this means they can basically Zerg-spawn disposable minions. In addition to their other spells.
