Dungeons and Dragons

If the character that was already dead, dies?

Nothing, they can't die. They're a Petitioner. They're in the afterlife. They exist in Arborea until they basically achieve Nirvana.

Going off what I recall from 2e planescape lore

Edit: I'm not so sure now, maybe petitioners can be killed. Hold up I have the box set right here

Edit edit: I'm not finding what I'm looking for from my books. But I think Petitioners can die, though I feel like they re-form on their plane until they're Done With Their Thing. The idea was that the Afterlife is a place you can visit, and the dead people there are trying to be "united" with the essence of their plane, sorta. You're NG so you go to the NG plane and there you work out your cosmic debt or what have you till you can just be a part of NG Heaven forever, and so on

My take is, it is your game. You could say the groups desire to return the player to life has caused their soul to be stuck in limbo whole they are looking for it, or send it to where ever they are going. If you want them to succeed then breadcrumb them to it. If you do not want them to succeed turn it into something big and nasty with a vengeance for them not saving the player.

Dealing with the afterlife and the planes is really up to you. I think making this a little unclear, vague and inconsistent leads to a little mystery to the game. In my game I would follow something similar to what you had laid out above. Here is how I would run this for my campaign.

• The players would have to find out where their soul went. This would be some adventure to find this out by petitioning Kelemvor or another method to ensure they find the correct location
• The players find a method to travel to the plane (I would do olympus because it would have some fun fights). I would either have some quest to get a planar aligned tuning fork (if they have plane shift) or some kind of planar tunnel to get there.
• Once there they will have to find the soul (who I would treat as a different creature for spells like scrying / divinations). I would have them continuously fighting and dying from the same fight.
• After they defeat this boss again, I would have some RP / skill check to convince the soul to come with them because their memories would be wiped / reduced. I would have time move very differently for the part vs this player.
• On the way back if the soul dies outside their fight arena I would have it dissolve into the essence of the plan (a.k.a. DEAD except for wish) or return back to their fight arena.

This sounds like a fun series of adventures.

Maybe treat them like a devil? They lose all experience gained in the previous iteration of existence and start again in their home plane.

Obviously I don't know your game, your table or your players so take everything I say with a grain of salt.

Why is true resurrection out of range but plane hopping isn't? This seems like a great opportunity to have them complete a quest for some high level NPC who does have access to true resurrection, possibly with motives that run contrary to the PCs? You can have the dead PC run another character for the duration of the side quest.

If they are already half way to the plane where this character's soul is, bringing the dead PC back to the material plane would revive them as far as I was concerned. Good luck getting them back though, also if it were my table there would be consequences to reviving this way.

Just as a side note it sounds like you are looking for the "correct" answer for what would happen in the forgotten realms, always remember you are the DM, correct can be whatever you want it to be.

Best of luck from a fellow DM! Hope you have a great next session, sounds like you are really invested and your players are lucky to have you.

A patron god creates a new body ex nihilo to enable the execution of an important mission. Maybe the death was part of a bigger plan?