[Build Advice] One of the PCs is secretly a powerful archfey

[Anaraxes]

Originally Posted by Donny Brook

For example, how will you convey to the other PCs that they are supposed to find the fey PC creepy after a while?

Change your descriptions of the NPC after a while (gradually or suddenly as the NPC may call for). In this case, you can also let them know it's not how they used to see him. Any decent player will pick up on the cues and respond. (After all, that's how they knew to respond to him as charming in the first place.)

Cases of enchantment or mind control are harder. The PC might have loathed an NPC, but after being enchanted, they think they're BFFs. The player, of course, doesn't have an abrupt rewriting of their memory, so they're not suddenly going to have a new view of the world. Again, a good player will catch on, but you can also just drop them a private note: "you've been enchanted. The Dark Lord is your new best friend. Please play along." The people I've gamed with are more than happy to play a twist or stooge a bit when needed.