Finished the campaign last night!! Huzzah! Such an accomplishment having a stable dedicated group, wow!
They heroes had been in and out of the Castle for awhile, having collected all the artifacts, but they did not at all feel confident of tackling Strahd himself. For a couple sessions I had them running scared through the castle, harassed by the vampire and his minions. Last night they were shown to the Chapel, where the Lord himself was speaking to a zombified Ireena (she killed herself AGAIN rather than marry the monster).
Final battle was pretty quick. Multiple uses of Counterspell, Daylight and Protection against Evil nullified most of Strahd's threat. The Sunsword struck him down and his mist form drifted through the floor. They group went down into the crypts, found the coffin, opened it and staked his heart! (I improved a little thing hiding behind a door, so when they opened it I was in vampire coffin pose. I opened my eyes dramatically and a player jumped in with the stake. Epic. No rolls needed.)
I told them the that the stake in the coffin pinned Strahd in, though he could still be seen he could not be touched. This disturbed the group as they realized that it was not possible to destroy Strahd, and that a miscreant merely needed to remove the stake to bring him back. After much discussion they decided to bring the coffin to Argynvostholt where they had established the silver beacon, and where they had been attempting to foster a community of good. They helped to rebuild the fortress. Izek (who was the Ally but had sort of disappeared into the castle) reappeared and offered to guard Strahd's prison. He asked for his devil arm to be severed, which the fighter did, then gave the sunsword to the grim defender.
The druid reincarnated Irena as a half-elf. The Abbot of Kresk came to pay his respects and tell them he was leaving to. Then the roc (which they had battled a few sessions earlier) landed and bore the team off over the mountains and through the mists.
I believe they want to go to Waterdeep, keeping mostly the same characters.
On the Strahd fight: I thought about having him retreat earlier, but finally had him dive into melee and his doom (he had been casting fireballs from the ceiling of the chapel). He was really depressed and I was kinda over it. Honestly, the campaign has been really satisfying, so no one was killed and the final battle was a bit underwhelming, but on the whole the end of the campaign was awesome.
Best campaign I've ever run, for sure.