Towards the end of kingdom hearts 3 and remind there’s hints that Vanitas is darkness like he says “what I am is darkness” and sora thinks Vanitas is speaking when darkness is. The foretellers are named after the 7 deadly sins and the masters sin would have to be pride “vanity” is related to the name Vanitas. Darkness seems to do a lot of jumping through time which could give him the knowledge the master of masters has. I have a theory that a split timeline was created in kh 3 and on one side sora becomes corrupt then takes on the appearance of Vanitas. After that, he then travels back to the age of the keyblade war and then he turns out to be luxu’s original body which gives even more knowledge potential for him to be the master. Then after all of the events of kingdom hearts at the end of kh 4 he travels back to the keyblade war as Xigbar/luxu and takes control as the master. Now in this state he’s the original master of masters we know and now he uses the power of waking to get around the kh 2 time and he uses yet another side vessel to watch the events in person. That vessel turns out to be demyx so now it turns out master of masters is sora vanitas demyx and luxu all in one and well that’s my theory thanks for reading.