Adventures where a friendly NPC becomes the BBEG?

[Kauyon]

Are there any published adventures where the PCs meet a friendly, helpful non-player character in early-mid adventure (preferably a long campaign) who becomes an important part of the story but then turns out to be the BBEG by the end?
For example, a friendly town wizard accidentally botching a complex spell and becoming possessed by a powerful demon, or an elderly tavern keeper discovering a tome of occult magic and going insane from all the forbidden knowledge contained within while gaining extremely powerful magical abilities in the process?

[James Jacobs]

Curse of the Crimson Throne comes to mind, but this sort of plot twist is one that's really tough and dangerous to pull off in an adventure, since players are naturally suspicious and the more you have "good guys turning out to be bad guys," you erode your ability as the GM to present actual allies to the PCs who the players won't trust. It's really difficult to get a group of players to start trusting your NPCs once you have a few of them turn out to be betrayers.

So we usually avoid it in our adventures, and when we DO do it, we do so in a way that it's pretty obvious that there's something strange going on with the "ally" NPC from the start. As seen in Curse of the Crimson Throne.