I think people are misunderstanding what a Real Dark Fantasy is.
It's a Fantasy which you do not really want because It can literally break you mentally, there will only be regret after it's over and you may or may not enjoy it while it is being played. So you can't really think about Your Real Dark Fantasies because your mind is not wired to produce them.
For example,
Your Wife is being fucked by those who have murdered your Parents and have raped your sister, killing her in the end. And the twist is that your wife was part of the Plan and now she's enjoying their dicks while you're literally dying Mentally, Emotionally and Physically.
Now, in this moment, If you choose to enjoy the scene in front of you and can literally become Normal again after it is over, you can reach a new height in your own being. You will do what is needed after it's over. But if you cannot maintain your mental balance then you're a gone case for forever.
(I think I may have gone overboard with this Dark fantasy stuff
)