This question came up when a friend of mine reported that his summoner had lost her eidolon in their last session of Rise of the Runelords.
According to him, the eidolon was killed in combat (which normally means you can resummon it the next day with half its HP remaining) while they were
Now I argued that his eidolon shouldn't be trapped this way for two reasons.
Firstly, because as a summoned creature, it doesn't die even when reduced to -Con HP, only its summoned form is destroyed and the eidolon returns to its place of origin.
Secondly because Outsider traits say that no soul is released when an outsider is destroyed, so the well would have nothing to trap.
Also the fact that there are no rules in the summoner class on what happens, should the summoner actually lose their eidolon. Whether they get a new one and how, or if they have to recover it or anything, so I would assume the only way to effectively lose an eidolon would be to retrain all your summoner levels away.
I don't know the GM, and wouldn't be able to tell how well they know the game, so I assume either of us could just be mistaken here. As far as I'm aware the GM let my friend just make an all new eidolon with which he seems fine, so I guess it's no harm done, but still I would like to know if it should have been possible in the first place to lose the eidolon.
Relevant rules text from the Eidolon entry. "The eidolon forms a link with the summoner, who, forever after, summons an aspect of the same creature." So as Volk said, you don't summon the outsider itself, you just summon an aspect of it.
Also even if technically this should work this way, the GM should not allow it to work specifically with the Eidolon. The reason being that when a character dies and has their soul trapped, you just make a new character. But if this happened to a Summoner Eidolon, you now have a living, but extremely gimped character that you now either are stuck with or have to get killed off purposely. Because by RAW you only ever get one Eidolon.
So i figured this much out, the eidolon was branded with a sihedron rune. And looking into the RotRL books I found that there is a
But even then it says that the soul itself wouldn't be affected any further and can be brought back normally. (if it had to, which it doesn't since the eidolon didn't die)
Or to look at it another way if you cast Plane Shift on player to send them to the abyss, they can't sent to the abyss.
If you cast Plane Shift on an Eidolon, it winks out of existence, it's a summoned creature that is no longer in range of the person who summoned it.
Anything that removes the Eidolon from the player just causes it to disappear.
From the summoner:
Eidolons are treated as summoned creatures, except that they are not sent back to their home plane until reduced to a number of negative hit points equal to or greater than their Constitution score.
From the magic chapter:
A summoned creature also goes away if it is killed or if its hit points drop to 0 or lower, but it is not really dead. It takes 24 hours for the creature to reform, during which time it can't be summoned again.
So yeah. Eidolons can’t actually die.