OK, I'm throwing out ideas here, but the more I think about this one, the more I like it.  So here's what we know:

 

* MX released his heart and took control of Terra's body

* At least a part of Terra remained with the body - we can see that in Blank Points

* About a year later AX released his heart again, splitting himself in two - a heartless and a Nobody

* When a Nobody is defeated, the body goes to darkness

* If the heartless is defeated before the Nobody, and possibly vice versa, the heart and body will merge and the person returns.

 

So when Sora defeated Ansem SoD, the heart (or hearts more likely) went to the Org's Kingdom hearts which most likely started by that point.  When the hearts in Kingdom Hearts were freed, I think that the universe "righted" itself and MX's heart went back to MX's body.  But Xemnas was also defeated, making Terra's heart and body free.  There is a very real chance that Terra have come back during DDD.

BBS Ultimania:

 

-- Was Master Xehanort the one who delivered the final blow to Master Eraqus?

 

Nomura: Because it was necessary to avoid showing such a cruel scene, it's difficult to tell from the presentation, but that's right. Terra saw this happen right in front of his eyes, so when he realised at the Keyblade Graveyard that Master Xehanort intends to harm Ventus and Aqua, this drives him to anger, saying, "Not only my father Eraqus..."* As an aside, although he says "my father", and we see from Eraqus' words that he thinks of Terra like a son, Terra and Eraqus are not actually father and son. Also, in his final moments Eraqus falls towards Terra - an important meaning is hidden in this scene.

 

-- In the secret event Xehanort says “The heart that resides within Terra’s heart.” Who’s heart is it?

 

Nomura: I can’t say just yet, but I’ve set it so that you’ll know who it is. Listen to Xehanort’s words closely, and read his reports carefully. I think you’ll know who it is then.

 

DDD Ultimania:

 

-- What happened to Terra's heart and body?

 

Nomura: In this game, Master Xehanort does not appear in the form of Terra-Xehanort when he transferred his heart, but as the old man he was before he took control of Terra's body. During the revival, what happened to Terra's body, and Terra's heart that was in opposition to Xehanort's control? In Kingdom Hearts, Birth by Sleep, it is to be considered that it was thought the whereabouts of the heart of Terra's master, Eraqus rested within his body.

This really doesn't prove anything other then the presense of Eraqus's heart within Terra's body. However many times it has been emphasized by Nomura, it has not been emphasized how effective it was at preventing MX from seizing control of Terra's body.In fact Eraqus has zero experience when it comes wielding or controlling darkness in fact compared to Master Xehanort (the puppet master) he's an amatuer at best.

Nomura also implies that something happened during the revival that enabled Master Xehanort to fully take control. He also places Terra's heart and Terra's body into seperate categories meaning that something that happened to one effected the other.

 

The way I see it there are two distinct possibilities.

• One during the existence of Ansem and Xemnas the two halves had gained enough darkness to allow MX's heart to completely consume control of Terra's heart and body(thus shaping it into the old man). Once they rejoined each other there was enough darkness for MX to have complete control. In this scenario it is likely that Master Eraqus is at the very least preventing Terra's heart from being banished from his body. This would explain Terra's confidence despite being told that eventually his heart would be engulfed by Master Xehanort.
• Two it has always been implied that when a part of Ansem had been left behind within the darkness of Riku after he banished Ansem from his body. It is highly possible that the part of Ansem that was left  behind was Terra's and Eraqus's heart. Bare in mind that after the events of Chain of Memories Riku locked the darkness within himself away. With the absense of the two when Ansem was destroyed Master Xehanort would be the opnly heart present at the time of the revival, therefore shaping Terra's body into the form of the old man. It should be noted that when Riku released the darkness within him in KH2 he took the form of Ansem SoD. By all the rules of KH this should have never occured unless a part of Ansem still existed within Riku's darkness. It should also be noted that once Riku truly accepted the darkness within him and decided to "consume the darkness, return it to light" did the memories of Terra begin to resurface.

There is evidence on both explanations that display why the old man is the one who returned. However there is no indication that Terra has seperately been recompleted as himself again. In order for this to be true he would need his own body back which is confirmed to be still under MX's control in some form. 

@devereaurx:

 

I'm actually done in this thread, but I have to clear something before going:

Why do you keep saying Eraqus lacks experience with Darkness? That's obvious, but irrelevant: His mastery of Light is what's implied to be aiding Terra.

Also, that Riku theory's immensely possible -- it would certainly be far more creative than scenario #1.

That's all -- peace out.

P.S.: How's your username pronounced, anyway?

Edited June 16, 20169 yr by Alan Smithee

I think Terra is simply locked inside Apprentice Xehanort, or his Heart is within Riku. I'm not sure of what to think. Oh, Nomuralogic.  Ahem... Terra's body is definitely at least one or two (Xemnas, Terra-Xehanort) of the Darknesses. His soul is apparently in the Lingering Sentient. As for his Heart... That's the main question.