I doubt we will see the Kings in RoI at least not directly, they seem to be saving them for later, but they are very much the puppet masters of the fallen, manipulating other houses to do their bidding and keeping themselves out of the spotlight and their goals a mystery. My guess has been that a warmind is involved with the SIVA threat but I don't think the house of kings is under the control of a warmind.
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I totally agree. I'm glad that we can build up to a huge showdown later down the line in Destiny 2 and beyond. I'd personally like to get more info on how they hid themselves from Oryx and SIVA in that expansion.