Oryx took Riven while the queen was away. She kept her own thoughts but was still under his control. When Oryx died she was left without a master and grew bored sat in her cage for so long watching the other taken drone about. When the guardian didn't claim the power from killing Oryx Savathûn came and collected it. She offered Riven the chance at a master/freedom (not too sure here) and Riven made it a Wish. Now Savathûn uses Riven and the dreaming city as a never ending power source for her worm :)