In Legion, you can use a mythic keystone to increase the difficulty of mythic dungeons to get better loot. How can I get a keystone to enable this difficulty? I've heard the last boss of a mythic dungeon drops one but I haven't found any official sources that confirm this.
You set a dungeon on Mythic difficulty and enter the dungeon through the dungeon entrance (not Dungeon Finder). Meaning, you need to gather a premade party of 5 yourself or join a party. Best way to find or create one is via the Group Finder in game and click on Premade Groups/Dungeons.
When entering the Mythic Dungeon, you will see a Challenger’s Pedestal. This pole regulates the Mythic plus. If you leave it alone it will be a regular Mythic run. Think of Mythic plus as an upgraded version of Challenge mode.
Completing a Regular Mythic dungeon will reward you with a key. This key will let you do the same Mythic dungeon on level 2 difficulty (1+). Only the same dungeon, no other one.
When completing the mythic 1+ (2nd difficulty) in a certain time, your keystone will upgrade to next level, which is level 3 (+2) and so on and so forth. Each dungeon has its own time point to be able to upgrade your keystone.
When completing a mythic difficulty in time, you will get a loot scaled for the level you completed on (see below). If you failed the timer, you will not get any loot, also your keystone will be depleted.
http://www.worldofmoudi.com/legion-mythic-plus-guide for more info