How to Unlock the Mythic Dungeons Two Boss

World of Warcraft's Battle for Azeroth expansion features ten five-player dungeons, with more on the way through patches. While eight of these dungeons are available in Normal, Heroic, Mythic, and Mythic+ difficulties, there are two dungeons that are available only at Mythic, and Mythic+, difficulty levels: Kings'Rest and Siege of Boralus.

Access to these dungeons must be unlocked before entry, through the completion of achievement and reputation goals. Each dungeon's story is tied to a specific faction (Kings'Rest to Alliance and Siege of Boralus to Horde) and unlocking the opposing faction's dungeon will require a bit more finesse.

How to unlock King's Rest on a Horde character
Horde characters will have this dungeon automatically unlocked upon earning the achievement Zandalar Forever!, which requires completion of major questlines in Horde zones. Many players will have earned this achievement by the time they've hit level 120 just from playing the expansion's leveling content.

The achievement requires completion of story quests in Zuldazar and Nazmir, as well as the Blood Gate questline, available from King Rastakhan after completing the two previous achievements. You'll also need to complete story quests in Vol'dun to earn Secrets in the Sands, but don't let the large number of quests in the zone scare you off. The actual storyline quest chain is not much longer than other zones. If your main focus is on getting the achievement for the dungeon unlock, you'll want to save all of those sidequests for later (no matter how charming those Vulpera are).

How to unlock Kings'Rest on an Alliance character
As both factions'leveling journeys take place on completely separate continents, Alliance players will not automatically progress towards unlocking King's Rest while leveling.

Players will typically be nearly halfway done with the War Campaign storyline by the time they hit 120. Upon reaching max level, a new questline will become available once a character has earned Friendly reputation with the 7th Legion faction. Additional storyline quests will unlock at various reputation levels, with the most important (for unlocking the dungeon) becoming available once 7500 reputation has been earned at Honored level with 7th Legion. Upon completion of this questline, access to Kings'Rest will be granted.

How to unlock Siege of Boralus on an Alliance character
Alliance characters will have this dungeon automatically unlocked upon earning the achievement Kul Tourist, which requires completion of major questlines in Alliance zones. Many players will have earned this achievement by the time they've hit level 120 just from playing the expansion's leveling content.

This achievement requires completion of story questlines in Drustva and Stormsong Valley. You’ll also need to complete story quests in Tiragarde Sound to earn A Sound Plan which, again, has a plethora of available quests (similar to the Horde's Vol'dun zone). You only need to complete those which are part of the main story, so you can save all of those sidequests for later when you’ve got the time.

How to unlock Siege of Boralus on a Horde character
Horde players will also have to work a bit harder to unlock the opposing faction's dungeons. Like Kings'Rest for Alliance, Siege of Boralus is unlocked by completion of the Horde War Campaign questlines. There is a reputation requirement for progression through this story. Horde players must earn 7500 reputation into Honored reputation level with the The Honorbound. Upon doing so, the questline beginning with “Operation: Bottom Feeder” will unlock and completion of this stage of the War Campaign will grant access to Siege of Boralus.

If you meet any difficult while unlock the Mythic dungeon, please remember that Speed4game can helps you to save time and effort to power leveling for your character. 

https://www.speed4game.com/WOW-US/BoostingService/Mythic-Plus-Dungeons/1097