When my summoner uses Magic Jar and posses another creature does her Eidolon disappear?
Eidolon dismissing effects based on the Summoner are listed as;
"If the summoner is unconscious, asleep, or killed, his eidolon is immediately banished."
So, your answer is; No, the Eidolon stays. (Until you encounter a condition that would normally dismiss your Eidolon.)
However, keep in mind any sensory limitations the spell places on the Summoner. Some can be over come with Shared Senses.
The main issue I see is that the summoner's body and the possessed body are in different locations, and the eidolon can suffer harm if it gets too far away from the summoner. Is the summoner considered to be at the location of his body or the location of his soul for that purpose?
The minimum lvl for Magic Jar is 10th (For a Summoner)
So your looking at a 200ft casting range per the spell.
Unless you also use the spell Unfetter, your Eidolon will have reduced HPs after 100ft.
As for what now counts as the Summoner's location, here is my take on it and why;
I believe the Summoner's location would flag as where ever the Summoner's soul presently is. Due to the rules of the Magic Jar spell being based upon the location of the caster's soul. Be it in the jar or host body.
This would call for extra caution when using this spell since the Eidolon could carry the gem used in the spell, and if it moves it out of range of your original body (per the magic jar spell limitations)... Well, that would be extraordinarily bad should the spell end out of range.
Mind you, the Eidolon could carry both your body AND gem or act as a guard for your body if mobility is not a concern. Its a powerful combination, but a very risky spell.
A far more abstract question to answer would be...
What happens if your Eidolon is carrying the gem and or body and is somehow dismissed? The Eidolon takes its equipment with it when it leaves...
Like I said, powerful...but very risky.
I had not thought of having my Eidolon carry the gem since she uses her claws to attack and doesn't really have pouches. I thought any gear on an Eidolon dropped to the ground? They way we understood it was the Summoner wears the magical gear and designates for the day either she gets its benefits or the Eidolon does. If Eidolon goes away, she can't just switch it over. Maybe that was a house rule I read somewhere. The Eidolon wears nothing, no gear besides what you imagine it to have on it. Unless you give it evolutions to use weapons and such.
The Eidolon and the Summoner 'share' magic item slots only so far as if both are wearing something that occupies the same slot the summoner's item is active. This causes the additional restriction of "I'll just take it off when I summon my Eidolon." Which works, but if you have an other that requires an attunement period each time you wear it you run into trouble. The Summoner cannot choose which items effect the Eidolon from the others they themselves wear. Also, items with an attunement period must be worn by the Eidolon for the listed time frame. This, unless you have a way to remain awake for that time the Eidolon must keep the item on.
Equipment that is intended for the Eidolon goes with it when dismissed. The same way as your summons that appear with weapons do.
You could always use Magic Jar to possess your eidolon, becoming a synthesist without actually taking the synthesist archetype.
Place your body in a backpack (or similar) and have the eidolon carry it. The focus stays in a belt pouch on your real body.
Summoned monsters appear and disappear with any equipment listed in their monster stat blocks.
How do we associate equipment that the eidolon did not begin play with to the eidolon so that that equipment does likewise?
Summoned monsters appear and disappear with any equipment listed in their monster stat blocks.
How do we associate equipment that the eidolon did not begin play with to the eidolon so that that equipment does likewise?
The stat blocks for eidolons are player generated.
By your reasoning, any equipment the player includes on the eidolons stat block appears and disappears with the eidolon.
Precisely. That is the difference between 'equipment', and 'here, hold this for me'.