The ability could be triggered, but I doubt it would be any clearer that way. On the other hand, forcing the attacker to choose between combat damage and permanent destruction could be fun, and would let you reduce the activation cost. I have to wonder how many people will be bitten by giving their opponents' unblocked creatures the same ability.

Mmm. Actually, it was always meant to be symmetrical, but I don't like the way (in either activated or triggered form) that the opponent will probably get the chance to use it before you can (assuming both players have 6 mana but not 10 mana), and the opponent will have a reasonable chance to blow up Naughtbringer itself. Given all of which, I think since this is a mythic I'm inclined to give it shroud.

...None of which is to discount your good suggestion of a way of reducing the cost. 6 was probably somewhat excessive anyway. But I don't think I like the triggered wording: it'd be something like "Whenever a creature attacks and isn't blocked, its controller may pay N and sacrifice it. If that player does, destroy target permanent of that player's choice." Either that or "All creatures have 'Whenever this creature attacks and isn't blocked, you may pay N and sacrifice it. If you do, destroy target permanent.'"

I have to say, it does feel a bit of a cheat to let a creature get in for damage, and then sac it afterwards. In the worst case, imagine playing a Ball Lightning with this on the board!

Hmm. Perhaps I should try to find a sensible alternative wording, then. I'll give it some thought.

What about granting the triggered ability? "All creatures have 'When this creature attacks and isn't blocked, etc.'"