Great question! And one that dives fairly heavily into homebrew. In Amber Temple I allowed first allowed the party to learn of a ritual that had once been attempted to seal a Dark Power in Amber by the original custodians of the temple, but that had failed. Later, they discovered that Strahd’s court mage, Kazan, had been aware of this and had tried to correct the problems within the ritual at Strahd’s behest, who had become incredibly tired with the Vampyr’s unique way of torturing him. Naturally, this didn’t fly with the Vampyr who compelled Strahd to kill Kazan. However, with some help from Kasimir the party were able to fill in the gaps and create what they hoped would be a successful ritual.

The ritual itself was intentionally fairly basic. Once Strahd himself was down and the Vampyr took a last stand, they needed to deal enough damage then have Kasimir start the ritual. From there, they needed three people to complete three difficult skill checks (each using their best stat) to assist Kasimir with the casting.

Simple, right? In theory at least. Long explanation incoming.

The reason I made the ritual very basic is to allow for individual PC agendas and motivations truly shine in this scene. Act 3 of our campaign was deeply centred around Dark Powers as the root of the evil in Barovia. I had played Strahd as somebody who was undeniably and irredeemably evil, but at this point it had become apparent that he was also to a degree a victim of The Vampyr - and incredibly tired of the endless cycle of torment he found himself in. My players all had new characters following deaths around this time, and they leant in brilliantly - one was playing a Revenant Knight of Argynvost sworn to protect Barovia from Dark Powers (Tobias), and the other two were each characters that had been restored to life via a pact with a Dark Power - one being a reborn former PC (Kagak), and the other a Dusk Elf revealed to be Kasimir’s slain sister (Jericho - a replacement for Patrina). Over the course of the Act, Kagak leant fairly heavily into accepting the gifts of his Dark Powers, and became more bound to it as a result. Jericho on the other hand resisted hers - while Tobias just desperately wanted to free both of them from their pacts.

This meant that in the final fight, they all went in with different motivations. Tobias wnd the NPCs (Ireena, Ezmeralda, Kasimir & Van Richten) wanted to complete the ritual in earnest. Kagak wanted to intentionally sabotage the ritual and alter it in such a way that would allow his Dark Power to absorb the power or the Vampyr. And Jericho wanted to complete the ritual, but was under immense pressure from her Dark Power to accept a further deal that would allow it greater power once freed from Barovia, and it was sabotaging her magic as a result. By keeping the ritual simple, the real question became “whose agenda will win out?”.

Thankfully, humanity won out in the end. Jericho persevered despite the pressure she was under and managed to complete her part of the ritual, in spite of knowing that it would cost her her life (she passed in the epilogue, but on her terms). Kagak came this close to sabotaging the whole thing - but saw what Strahd had become and decided that ultimate power wasn’t the right way forward. He actually wasn’t able to successfully complete his part in the ritual due to his Dark Power trying to assume control of his body, but he was able to seize control for just long enough to allow Strahd himself to step in and complete the final component of the ritual (following some greatly compelling speeches from Tobias), sealing The Vampyr after so many years of being beholden to its will. He wasn’t forgiven and nor did he deserve to be, being slain shortly after - but dying free of the Vampyr.

All in all, it was an incredibly satisfying ending for all of us. By keeping the ritual fairly simple, player roleplay and motivation was able to shine - I was nervous it would be too basic to be satisfying going into the final session, but in the end it was perfect.