Quote Originally Posted by MultiBear View Post
They do not. The difficulty of Legion's Mythic dungeons is determined by the keystone you use. There's a base difficulty, a little above the Legion heroics, and after that they simply get more challenging based on the level of your keystone and whatever modifiers are active at the time (e.g. giving mobs extra health, things like that). Chances of better rewards increase as the difficulty increases.
Oh, a linear difficulty level that you can choose as you wish? Then it's much better than I had assumed! Thanks for clearing that up