In short, Rhiannon is a riftwalker, a rare type of dragon capable of walking between dimensions. Her father, Lord Necronis, was one of the founding members of the Order of the Void, and the third and final dragon in the group. He was one of Void Lords who fell during the human uprising when the Void Lords ruled the planet.
He was ruler of the dead (an army that Devilak claimed after his death) and was a fearsome, brooding dragon with an affinity for necromancy.
Now, a thousand years after his fall, his daughter, Rhiannon, wants to ressurect her king and mate, Nefarion, by sacrificing the souls of the living to revive his own. Thus, she enacts her plans upon the three kingdoms of the world, but has yet to do more than speak to the Void Lords in their lands. Her first target is the Archon's lands.
She holds a grudge towards Devilak, for his claiming her father's army, which she believes is rightfully hers.