From What I remember, when you play as aragorn, after he becomes a king of Gondor, you get an event which choses what spouse he can have. I

Now here is the kicker, most people probably will have only choice between arwen, Lothiriel and Eowyn. However, If you play as denethor and expand Gondor so that you have diplomatic reach with the Belakar, Aragorn will have an option in that event to marry Belakar king's second daughter. After few years, you will have option to vassalize Belakar through another event.