iserith
Magic Wordsmith
The rules exist to say how the game is to be played. That applies to player and DM alike. The DM just has the authority to change, add or subtract rules if he wishes. Those are house rules. Outside of a house rule or ruling on something that is vague or the like, the DM has to follow the rules. What you are suggesting is that the DM house rule damage which is always uncertain as a general rule and only becomes certain if specific beats general, into something else, allowing him to declare uncertain damage certain when he wants to.
You sound like you're talking about a different edition of the game.
If you have to re-define what a ruling is in order to be right, there's a problem. There's a reason why people hate bench legislation by judges when they go beyond making a ruling and into the territory that you espouse here for the game.
I think that how I define "ruling" is more compatible with the D&D 5e paradigm than what you suggest. I think yours is more appropriate to another edition of the game. Which game did you play the most before D&D 5e?
I know the rules, so I don't really have to seek for one. What the player wants doesn't matter as far as the outcome is concerned unless the PC has done something to eliminate chance or just has no chance at all. That doesn't apply to damage, though. There's nothing a PC or goblin can do to prevent an arrow from killing someone once it is in flight.
What the PC or goblin wants is irrelevant here - the DM set the stakes. The failure condition is unconsciousness and robbing. That is the result of the goblin's successful attack roll against the 1-hp wizard.