Mythic Keystone Dungeons are the hardest form of 5-man content currently present in World of Warcraft: Legion. Their scaling difficulty is more than enough to provide a challenge for even the most seasoned of veterans, but at the same time they offer some of the best gear you can possibly obtain, so its definitely worth giving them a shot!
If you're wondering what these Mythic Keystone Dungeons might look like, and whether this is something you can even attempt yourself, here's brief gameplay video that showcases how me and my group managed to demolish Maw of Souls so hard we ended up with three chests at the end. In other words, don't worry about the difficulty and just throw yourself in there - its pretty fun! Anyway, here's the video:
If you're interested in my full, detailed review and critique for World of Warcraft: Legion, you can find it by heading over here. I hope you enjoyed the video!