All Terra knew was unrelenting darkness, for that had been all he knew ever since control of his body had been usurped from him by Master Xehanort. So here he was, a heart made prisoner in its rightful body so the will of another could exert control over him. No matter how many times he tried to fight against Xehanort, the elderly Keyblade Master always retained control.
And it didn't help that the heart of Master Eraqus had disappeared when Xehanort plunged his Keyblade into the chest of his new vessel to get rid of Terra's heart permanently during the fight with Aqua in Radiant Garden all those years ago. But it didn't work like he anticipated and the two hearts were both vying for control of their shared body.
While Xehanort had been the dominant presence in the creation of Ansem and Xemnas, Terra did manage to exert some of his traits into the Heartless and Nobody. This manifested when Xemnas referred to Aqua's Keyblade and armor as 'friend' during his visits to the Chamber of Repose and his search for the Chamber of Waking.
But whether he was influenced by Xehanort to find Ventus and use him as a pawn, or whether he was influenced by Terra to save him remains unknown. His thoughts then drifted to Aqua and Ventus. He hadn't seen the two of them in so long. The last time he remembered seeing Ventus was when the young Keyblade Wielder had been forced to join with Vanitas.
As for Aqua, he vaguely remembered plunging into the darkness. But he was sent back to the Realm of Light while she ended up trapped in the Realm of Darkness. Terra then thought about Sora and how he was holding up after his near possession by Xehanort. Although Terra tried to believe that he would be rescued from Xehanort's control someday, he was losing hope.
He then remembered the promise he made during his last moments in the Keyblade Graveyard. "Aqua, Ven, one day, I will set this right." The words echoed in his mind over and over again. Terra then felt a sense of renewed hope flooding his heart and he looked into the darkness.
"Even if I need help from Sora to do it, I will set this all right. Aqua, Ven, Master Eraqus, I'll be waiting." Terra's ethereal form then disappeared altogether, but not before he smiled one last time.