As the only 1-cost Eagle, this is highly desirable for expensive Eagle decks. It's not half bad outside of them either - as a matter of fact, the only other 1-cost ally in all of Tactics is Knights of the Swan, who's useless on his own. Vassal is a nice damage spike/potential chump blocker for little investment, in a sphere that tends to lack money. Also a good target for Support of the Eagles when you don't have time to build up an Eagles of the Misty Mountains.
Vassal of the Windlord
Ally. Cost: 1. 0 3 0 1
Ranged.Vassal of the Windlord cannot have restricted attachments.
Forced: After an attack in which Vassal of the Windlord attacked resolves, discard Vassal of the Windlord from play.
The Dead Marshes #98. Tactics.