I finally figured it out! Thanks to Nitehawk! See, since Xemnas and Xehanort's Heartless both died in Kingdom Hearts (the Realm, not the name of the series) then they were absorbed by it. Thus, the Heart and the Body&Soul were reunited, giving birth to the Complete Xehanort. The dematerialisation of the Dragon Nobody represented either defeat or absorbance, but, either way, right after it disappears, there is a sound and the Three Knights stand watching a new being (the Shadow Enigma) coming at them. Who else could it be, that fits, I mean? MAYBE it's Zexion, but this makes a lot more sense to me.