Hold it!
Here's my theory: Based off of you saying them being exes, here's what I think.
I think that they were together, but then over time, due to Marruk's constant mood swings, and Rhoun's, uh... Tendency to space out a whole lot, along with him being somewhat scatterbrained, those two just sorta fell apart. Marruk felt like he was to blame, and neither knowing what to say to him, nor what to do about it, he tends to lash out the most on Rhoun, because he's the one who seemingly "hurt Marruk".