Didn't she get killed at some point after the events of Wrath, and Garrosh told the Val'kyr, since they were already there, to bring her back, and died in the process? That way Sylvanas was more convinced they're willing to truly work for the Horde and the Forsaken? Edit: Or see quote

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Wowpedia's Sylvanas article
High Warlord Cromush orders the val'kyr Agatha, Arthura, and Daschla to fix her. After a moment of thought the three val'kyr sacrifice themselves and give Sylvanas life once more. This convinces Sylvanas beyond a shadow of a doubt that the val'kyr are the future of the forsaken