Didn't find any thread about this, so here it goes:
At the end of the worgen starting quest-line Lord Godfrey, a gilnean noble will be raised by the val'kyrs, and becomes an undead.
Later he betrays Sylvanas, and kills her. After this, he flees to Shadowfang Keep, he will be probably a boss in HM. I think Sylvanas wants revenge on him too, just like on Arthas. One of the val'kyrs brings back the banshee queen to her third life, at a price of her own life.
With this the val'kyrs proved how loyal they are to Sylvanas.
Screenshot: http://img514.imageshack.us/img514/1747/sylv.jpg