Get Up To Speed with Legion

The Tomb of Sargeras has been reopened, and the demons of the Burning Legion pour into our world. Their full, terrifying might is fixed on summoning the Dark Titan to Azeroth—and they’ve already located the key to his return" (Blizzard).

At the end of Warlords of Draenor, Archimonde sent Gul'dan to present day Azeroth through the Draenor portal, reminding Gul'dan of their pact. This pact was most likely a plan of the Burning Legion to invade Azeroth. Gul'dan is sent to open a gateway from Azeroth to the Legion. It is revealed that Kil'jaeden gives Gul'dan information once he arrives on Azeroth. Gul'dan, using magic from Draenor, opens the Tomb of Sargeras.

Tomb of Sargeras, by Dmitry Vernygor

The Tomb of Sargeras was created by Aegwynn to lock away the Avatar of Sargeras and is located somewhere on the Broken Isles.

The Broken Isles

The Broken Isles (this is debated whether Gul'dan raised the Isles themselves, or just the Tomb of Sargeras) were risen by Gul'dan in Warcraft II to an effort to enter the Tomb and find power in it.

Highmountain Tauren

There is much speculation as to how the Highmountain Tauren, a new race of Tauren native to the new Isles zone Highmountain, could have lived there sunken under water. Some speculate Aegwynn only sunk the Broken Shores, which may house the Tomb of Sargeras. This would play in well to the pre-launch Broken Shores event. Some speculate because Highmountain is so elevated, perhaps the actual mountain was not sunken, allowing the Tauren to live there. But then what about the Vrykul in Stormheim?

LoreCraft's take on it? The whole of the Broken Isles was not sunken. The Sundering only sank Suramar, meaning all the other areas of the lost bit of Kalimdor were left above the waves. So how did the mortal races not find it yet? Chance. They have been busy fighting in Outlands, then Northrend and the Lich King, then Deathwing, finding Pandaria, and their latest exploits on Draenor. It seems wherever one goes, the other hastily follows, scared to be left out. Countless years of war and bigger issues could have led to their not having found it yet. Besides, the location of these areas would be kept a secret. If you fought to contain the Avatar of Sargeras from destroying the mortal races and Azeroth, wouldn't you keep the location of his prison a secret?

Illidan Stormrage

But that is of course all speculation. Moving onto the next bit of lore, we see Illidan Stormrage make a return in Legion. Illidan was imprisoned in the Vault of the Wardens by Maiev Shadowsong. However, being on the cover of Legion and being in the Gul'dan cinematic shows that Gul'dan is releasing demons and evils from the Vault of the Wardens as an addition to the Legion forces coming in through the portal. Perhaps Illidan can come back to good and redeem himself later on in the story?

Nobbel87 also has a great video on Legion Lore as well that goes more in depth for those looking to go more in depth.

Sources Used:

http://wow.gamepedia.com/Broken_Isles

https://www.youtube.com/channel/UCX34tk-noBVC4WVC9qQGyMw