Use your Enthralled Val'kyr's Call to Arms ability to mobilize 5 Forsaken Troopers on Felstone Field. Then slay the Opengut Abomination inside Felstone's barn.
A val'kyr's only purpose is to serve, and this one serves you. I am now giving you full control over her abilities, <name>.
Roaming around the farm's outskirts, you should find several of the Forsaken that you just created. I want you to use your val'kyr to gather at least five of them. Once you have done so, take your newly-formed band of soldiers, bring them into Felstone's barn, and kill the beast inside. Today, they will learn what it means to fight for the Dark Lady.