Quote Originally Posted by Cluedrew View Post
But to do that aren't you messing with the control of there own body? Why not just turn it off completely and kill them then? Or why not use that to rob other wizards of there ability to cast spells?.
I feel like a number of things in the quote by Quertus, and suggested by you, can be achieved with Bestow Curse in 5e. Assuming the creature has no more Legendary Saves (which I think are just Portent Dice with DM fiat on the number rolled).

Bestow Curse is a much lower spell than ones that might be able to “instantly kill”. Maybe uses of Hex, or Wish, or other spells, can also be used to debuff a Dragon until a whiny-Player is happy with how awkward the Dragon is acting.