Originally Posted by
AwkwardSquirtle
The Legion probably weren't aware that the Night Elves were immortal, and even then it was immortality in the natural sense, they couldn't die of illness or age, but they could still be killed in battle. They were invading for the same reason as always. Previously it was to conquer and destroy, and to get revenge for the War of the Ancients in part, now we know it's to destroy Azeroth's World Soul to protect her from the Old Gods turning her into a manifestation of a Void God.
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If the invading army travelled through the Great Dark Beyond, Sargeras wouldn't necessarily travel with them, just as Kil'Jaeden hasn't (to my knowledge). He'll wait for them to summon him, because he can, just as he did during the War of the Ancients. I've got no proof that he isn't, I've just never been convinced that he's trapped in the Nether, never seen convincing evidence. My point wasn't that he isn't in the nether, just that he doesn't have to be.