You decide to do it in EXACTLY the way they wanted you to, but you/your character didn't know that the attempt was always going to be futile.
How should a tabletop DM handle something that?
Exactly as it was suggested at least twice in this very topic ...
Simply dont let people roll.The whole scene we had here could be litteraly exactly the same ... minus the roll ...
1. Dominate the brain.
> cutscene plays out.See? Its not that hard.
If you include roll there, you are telling your player "hey, you actually CAN affect this outcome" ... they can spend spellslots, potions, inspirations and other stuff ... in order to gain that "different" outcome you just promised them BY LETTING THEM ROLL ... and all they get as reward is exactly the same shit.
There is difference between:
Player: I want to persuate King to give me his kingdom.
DM: No, you cant do that.and
Player: I want to persuate King to give me his kingdom.
DM: Okey, roll for it.
Player: Nat 20!
DM: Yeah ... no, you didnt.