For all those saying that Terra being one the current 13 Darknesses defeats the purpose of his confidence in BBS's secret ending. Did you not hear what Terra told MX, "Even if you do wrestle my heart from me. Even if you cast me into the darkest abyss. You'll never sway me from the one cause that keeps me fighting. Whatever the cost i'm willing to pay it." In other words Terra knew MX would probably go to extreme lengths to carry out his plans, Terra knew it was a possibility that he could be put through years of terrible things MX could do to him (Like becoming a heartless and nobody), but that in the end he would stop MX. As Terra said, "You're going to get shown the door, old man."
When dealing with someone like MX, sometimes you have to go to just as many extremes as he does, and to patiently wait just as MX does, in order to find a way to out smart and beat him.
Terra is one of the 13 seekers of darkness right now. But by no means does that mean that Terra has lost to MX. I'm sure MX isn't the only one with backup plans in the works. Terra has Eraqus' heart inside of him, and therefore Eraqus' light. There is an escape route for Terra in all of this madness MX has done to his body and heart, but what that is remains to be seen.
This type of planning isn't uncommon in stories. Mostly villains are known for this type of extensive deception, but heroes have done it as well. It's like a game of chess between MX and Terra, even if Terra isn't directly in control of himself, he would have had someway to ensure he could take control of himself again and force whatever remains of MX in him out.
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Sometimes a hero has to play the bad guy, or allow the bad guy to use him or win a few times, to ensure the bad guy loses the war. I believe this is what we are seeing with Terra.
Terra is not Aqua. Aqua has certain lines she is not willing to cross over, even if doing so would stop MX. Terra on the other hand is willing to cross certain lines if it means stopping MX in the end. When Terra said he would pay any price to stop MX, I think we all damn well take Terra literally. Nothing is off the table when it comes to what Terra will do to stop MX (Except for going after his friends of course, since that would defeat the whole purpose of what drives Terra to keep fighting MX)
In KH3 i'm sure Terra is going to get his moment when he's freed of MX and probably has done something to really screw up MX's plan and MX is like, 'You mean you..." and Terra gives a slight victory smirk, "Yes, from the very begining."
And and by the way, Terra and MX's heart fused together to create one heart when MX took Terra over in BBS. It wasn't two seperate hearts inside of Terranort. It was one heart created from a fusion of their hearts. Terranort and Sora are different in the whole having multiple hearts inside of them. Sora's heart isn't fused with Roxas, Xion, or Ventus' hearts. Sora's heart is housing their dormant sleeping hearts, but their hearts are still seperate from Sora's heart. The proof of this is that Sora has not had any physical changes to his appearance despite their hearts being inside of him.
In the case of Terranort, Terra and MX's hearts were fused. MX wanted and believed he could use Terra's darkness to consume Terra's heart. But Terra's heart was more resistant than he predicted. The result was a single heart, created via the fusion of Terra and MX's heart.
Since the awakening stain glass pilars are the inside of a heart and the image on the stain glass reflects the state of that person's heart, then Terranort's heart should look like this at the end of BBS, just imagine an image of MX on one side and Terra on the other, one awakening so one heart, but created via fusing of two hearts. Terranort's awakening probably looked like the one Ventus and Vanitas fought their final battle on (One side of the pillar being Vanitas, the other side being Ventus). Showing MX and Terra still in conflict. But obviously since MX has seem to gain almost all control over Terra, and even though he is no longer in Terranort he still left a fragment of his heart in Terranort and a damn powerful since that was his direct vessel for a period of time. So in KH3D Terranort's awakening probably shows Terra with white hair, tan skin, ect, but their is problem something like a wayfinder symbol or 3 wayfinder symbols on his awakening to show there is still a part of Terra that MX will never be able to control.