Wicht
Hero
What I mean is for instance let's take regeneration. Is d&d its a rule, regent at a certain rate at the beginning of a turn. Break those rules, it regens whenever seems appropriate. Or don't give the monster HP, it doesn't play by those same rules.
I think thats just called DM fiat. While sometimes it can be useful, I think its a bad habit to fall into personally. If the players think or realize you are being arbitrary, then fear can turn to frustration. At the same time, if something happens because, "its in the script," even if there are no actual rules for it, I think its fair (though beforehand I do try and rationalize a way to make it work within the framework of the rules.)
However, tell me how you have done this and how it worked for you?