the final boss of Emerald Nightmare, the 1st raid dungeon in Legion.
For killing Xavius Mythic players will receive 880+ item level piece of gear. The Dreamer title is rewarded to players who are able to defeat Xavius Mythic. You can buy Xavius Mythic kill and share your account to our pilot, if you don't have time to play or any other reason. But you can always participate in kill by yourself.