How to handle a mega magic mash in Fay Jee ?

I'd like to have your view on my campaign's next twist, and how to turn the upcoming event into something memorable, a high point ("watershed" ?) in the life of Fay Jee.

Following Orlanth's death in faraway Whitewall, Fay Jee the demon, an avatar of Baraku (aka Mastakos) lost his marbles and let loose in the city streets hosts of ghosts, which he normally holds back.

There was some warning and the city had time to go in blitz mode. All congregations took refuges in their churches / temples to unite in powerful prayers of defense. However, the PC had the time to tour them before and convince them to unleash powerful exorcism rituals to free the ghost victims of the curse of Ygg placed on them by Harrek when he levelled the city three years ago.

By some strange twist, the PC had to go out in the streets and mingle with all these ghosts (brithini, zaranistangi, harrek's victims - you name it). Then they were bushwacked by the True Prince, ghost of the former ruler of the city, who had been ousted by the present prince and believed killed. He's in fact become a ghost sorcerer of great power mastering the mysteries of the False Sun. He had a grudge to settle with the PCs and blasted their boat, sending them in the drink.

Here we are : PCs in the drink, one of them out, and several hours to run before the exorcism rituals go off.

I want to make this a key moment : at some point, the PCs will have to choose between using the exorcism's ritual energies to either free the ghosts or blast the True Prince into oblivion. It's a difficult choice, both answers have positive and negative consequences. They will have absolute freedom to choose : there's no "right" answer.

My question is, how do I make this moment memorable ? What gimmicks could I use so the PCs are in position to channel the exorcisms' energies ? How can I "plug" them into these huge flows of energy sent by different cults and sects ? How do I make it look and feel dramatic, intense ?

I already have a few half-formed ideas, but after reading your great ideas about how to sneak into a rokari ceremony, I'd be curious to know your take in this situation.

Oh, BTW, the rokari "sneak-in" will come after the ghost mess. If they survive it...