I'd keep it simple and relatively rules-free if I were you. Next time you play with your party set a TPK for them. I'd prefer a nice setting which would make the transformation plausible. The spellcaves of eryndlyn comes to mind...spellplague, plenty of faerzress, a powerful earth node and close proximity to the high moor and perhaps a dark disaster effect still lingering down there.
Combine their hps and let them play normally, in every init count this player's personality takes charge. It may be fun or a total fiasco but it will surely be something new. You should encourage your PCs to tell aloud what they intend to do and let others voice their opinions. If they seem like dragging a lot start reducing their init results something like delaying. Multiple personalities are confusing but sharing everyone's opinions should be both a boon and a curse.
You can creep them with visions that run a broad range of emotions. From noble thoughts of saving the world as an ancient mage trying to stop the killing storm from engulfing faerun, to unimaginable suffering as your spellscarred body convulses while your superior intellect searches for ways to unleash your demiplane to toril and ease the suffering by sharing it with the rest.
Make these intrusions a taxing mental skill challenge as your party tries to shut off the myriad personalities, That could explain why they cannot utilise their sharn abilities, something that could be achieved only by losing themselves.
I would try it for 1 adventure. Finding clues as to what they really are and perhaps travelling to rymanthiin to seek a solution. This could be a good way to present them with clues such as the sinister designs of the order of blue fire esp if they believed it to be a bening organization so far.
Encounters will include the aforementioned order. They could get on their knees and await your orders or make a crusade to kill a creature whose very existence mocks the "chosen ones". Agents of netheril would love to capture a creature that managed to capture the phaerimm. I suppose anyone knowledgable enough to know that you are a weak specimen of a creature that could provide answers to a thousand ancient questions could fill the bill as well.
Hmm, even if they make it to rymanthiin, would a spellplague-touched sharn qualify as a creature that harbors no ill to the hidden city?