The 8.3 storyline reveals that Azeroth (the titan) is unrecoverably corrupted by the Old Gods and that the only solution is killing her, which would leave us with a dead world.
This, it turns out, has been on Sylvanas’ radar since the very beginning. After the Champions kill the Titansoul, she absorbs all azerite on the planet into herself and ascends to become the new World Soul, purging the old gods from the planet entirely. In doing so she attains the truest immortality she could find, but as a nascent ‘titan’ is forced to sleep while she grows into her new form.
N’zoth is effectively killed offscreen (lmao dunked) and Nathanos becomes the new Forsaken leader, with the Forsaken now giving thanks to the Titan Queen.