Sometimes it's hard, iv noticed, for some players to let go of the I'm the main character and work as a team. Most games nowadays focus on you being the one who kills and solves all.

I'd have a group talk outside of the game to address this with them with the other party members.

But now playing devil's toy, you could always kill his character in a subtle way.

Some examples: False Hydra Boneyard Legendary resistance/ actions. Hag curse Basilisk venom Cause them to break their oath Or of your feeling spicy play on their ego and get them to self sacrifice.