My head canon is that Theta and Koschei were comfortable and happy together, but Koschei steadily started losing control because of the drums and became possessive and abusive towards Theta and in the end Theta left because of that. And Theta leaving would've cause the connection they had in their mind to break. Please R&R
The Fire of his Mind
Theta clutched his head tightly with both of his hands, covering the majority of his face as he did so. His fingers were buried deep into his light brown hair, pulling desperately at the roots. His eyes were scrunched up as tight as possible and tears were still managing to fight their way out and line his cheeks. His past scream was still echoing through the endless, empty, lonely space of the TARDIS, although he was silent now. He was curled up on the cold, unfriendly ground, as small as he could possibly make himself, his arms and legs slotted in together like a jigsaw pulled together. His breathing was heavy and jiggered, a desperate attempt to act as though he had some sort of control over what was happening. As though if he could control his breathing, keep it steady and self then he could do the same to his mind.
It didn't work like that. The fire in his mind continued to burn, it had grown stronger and stronger, destroying everything it could in his wake. Theta had no way to stop it, no way of making it better. He couldn't think of anything to try and calm himself down because natural he would think of Koschei, but Koschei was exactly the problem here. It was the memories of Koschei and him that were burning like wild fire in his mind. The fire seemed to walk its way through their relationship, picking out the key moments and destroying them. It left them there in Theta's mind but blackened by this break.
There was no telling how much time had passed before Theta had stopped screaming or how much time passed while he was curled up desperately. There was no way of telling how long it had been before Theta's fragile body began shaking as the mental, emotional pain became physical, unbearable. He whimpered quietly to begin with before he started begging, "Please," he whispered, tears pouring down his cheeks like a river, "Please stop it, stop it, stop it."
He repeated the words many times, talking to whoever had the power to make this go away. He knew no one did though. He knew he was completely alone in the universe. He knew that all this pain he was feeling was his own fault. He knew he probably deserved it. But it was unbearable. Theta needed it could stop, he couldn't cope with it.
He cried and begged and whimpered until the fire stopped raging through his mind and the moment it did he passed out. Neither his mind nor his body could deal with the pain of being conscious after that level of pain. His unconscious state could not be described as peaceful or restful, but it stopped him feeling for a while.
When Theta woke his mind was in disarray. Everything was completely still and nothing hurt, and yet as echo of the raging fire remained, giving the illusion that it was still there burning. His fingers unlocked themselves from his hair and slowly he sat up, his head spinning as he did so. Once he was sitting it took him a moment to realise where he was, why he was feeling so completely shocked and hurt and numb. The TARDIS he had stolen, his TARDIS. On his own. He shivered slightly, aware of how cold it was around him, aware of how he no one in the universe and unaware as to why he had ever thought was a good idea.
It became warmer around him almost instantly and Theta forced himself onto his feet. He took a couple of wary steps towards the console and leant on it. The design of the console seemed to be taunting him, six sides, for 6 drivers and he was alone. The TARDIS itself seemed to be working with him. He had done nothing and yet it had given him the warmth he had needed.
A sound came from the TARDIS, which could only be described as a grumble of annoyance and Theta thought for a moment before speaking aloud 'Not it. She?' The TARDIS made another noise, in agreement this time and Theta gave the tiniest of smiles.
It didn't last long. His mind kept moving back to Koschei and whenever he did it hurt. It was as though his mind was aware of the damage that had been done by losing the connection with Koschei and now it was trying to prevent any further damage by trying to removing Koschei from there. What his mind's natural defences didn't seem to understand was that Koschei would always be there. And even if Koschei was the reason for the pain, Theta knew he himself was equally as responsible. He had made the decision to leave, the decision which had left him completely alone in the universe.
The TARDIS gave a little hum and Theta couldn't help but amend that in his head, almost completely alone in the universe. He had the infinite space of the TARDIS and the universe and no one to share it with.