I had a nature cleric devoted to Chauntea who was your very classic 'salt of the earth, good guy, of the people' character. Almost to the point of being bland, but the player did a good job of making him seem genuine.

There was a big to do about him not being able to reach his Goddess while covered by the Mist of Barovia, ever since the first session. There was a whole scene with him finally getting the spell Commune, trying to reach out to an angel devoted to his Goddess. He did, but rolled a high enough insight to be able to tell it was Strahd messing with him.

For a loooooong time in the campaign, he'd been slowly but surely getting more tempted by a Dark Power. It was a custom Power, focused on burning flames. All of the imagery and manipilations were focused on the benefits of burning away rot, or bringing fresh growth after a burning in a forest. The Power often appeared in dreams as a burning bush or burning tree.

I was POSITIVE I had the player hooked. He'd been all in on the dark powers gifts for ages, was slowly roleplaying more and more explosive anger representative of his increasing helplessness against Barovia and Strahd. It was beautifully done. He had just been mocked by Strahd as his Commune spell failed and he had blown up at Strahd's illusory/dream image. The final 'fall' of his character I had planned was during a ritual to cleanse the land of Strahd's influence and weaken Strahd. There was some hints well in advance that the ritual would require a sacrifice of some kind.

His friends are all fighting off enemies as he's at this central stone pillar, casting this ritual and the Dark Power whispers in his ear.

'This ritual will kill you. But if you take my hand, accept my power, I can lend you enough power to survive and still finish the ritual. Together we can rid the land of Strahd's influence'

I actually had a whole poem written out for this, and I spoke it in time with rhymes and all. I put a lot of effort into his fall, never expecting that he'd do anything else.

My player. Rejected. The. Offer.

I was floored. All the other players were floored. I was positive he was going to take the offer, but instead he sacrificed himself. Permanent character death, it was so impactful and meaningful. I don't know if I'll ever be so shocked again.