The final boss of Legion is Sargeras, from the following voiceline:
The titan soul of Argus is the key to breaking the Legion. Our defenses must hold long enough to destroy it... or Sargeras wins.
Perhaps this world's past can aid us. I will scour my memory for what may yet remain.
Ye did it! Aggramar's essence is free! He can stand beside his brethren once more.
The very cradle of creation!
The titans can sever that connection... but not here. They need ta bring the soul o' Argus ta their place o' power... the Seat o' the Pantheon.
Our victories mean nothing. So long as Sargeras has the soul of Argus to fuel his infinite army, Azeroth remains in grave peril.
Their final act will harness the last o' Argus's power, to imprison Sargeras... once and for all.
Heroes, I can signal the titans ta begin. Once they start their ritual... there's no turnin back!
The tortured soul of Argus has been put to rest.
It matters not. We have lost. Look to the skies!
Sargeras will soon undo all we have fought for.
No.
We will use the last glimmer of Argus's power to bind him here. The Seat of the Pantheon shall become Sargeras's prison... and ours as well.
You would condemn yourselves to stop him?
Our world must survive... no matter the cost!
A sacrifice must be made.
Return home, children of Azeroth. Protect the final titan.
Prophet... what happened out there?
Illidan serves as the Dark Titan's jailer. His sacrifice has ended the Legion. At long last, the Burning Crusade...is over.
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