The Sylvanas fight ends when you get her health to 50% on Normal and Heroic modes. There has been speculation that you may require a lower health for completion on Mythic, supported by an aura datamined by Wowhead: Sylvanas.
Continue reading »
By now you have likely heard of the surprise surrounding the end of the Sylvanas fight during the Race to World First for the Sanctum of Domination. If not, here's the situation: