Since PFS has banned the Synthesist class, I have been considering solutions for playing my main character in a manner similar to how I had played before. At first I thought of just playing the character from the point of view of the Eidolon. The only difference being that the Eidolon now has an elf following her around that she uses as her scale polisher.
Then I noticed Marionette possession, which is a great solution for synthesizing if you don't mind using all your high level spell slots on extended possessions, as well as carrying around your helpless elf body. My Dragon girl was already known for carrying around giant statues of Irori as part of her strength training. Thus it would be nothing out of the ordinary if she was carrying around a thousand pound "Huge Cage" re-skinned as a shrine of Irori, with a helpless elf body hidden inside.
Looking through the new Ultimate Equipment book, I found what might be an even better solution that would leave all spell slots intact. The "Summon-Slave Crystal". It is a 10,000 gold item. I am wondering if it won't work however. The text says "When casting a Summoning spell, the caster may immediately possess the summoned creature without a saving throw until the duration of the spell expires." Summoning an eidolon the standard way is called a "ritual", so this may only work for the 2nd level spell that brings the eidolon into play for a few minutes.
My question would then be, would this item be the ultimate synthesist item, and if it doesn't work, what are some other solutions people have come up with?
By analogy to Augment Summoning, I'd say you're right - it only works with the spell Summon Eidolon, not with the usual calling ritual.