Disclaimer: By the power invested in me, I now pronounce myself unable to claim Doctor Who.
Foreword: This is a stepping stone to better writing, I swear it is.
Much-abbreviated prologue: Theta and Koschei interfere, and it sparks a civil war.
Koschei touches the back of his head, and his fingers come away wet with blood.
There's a voice saying something urgent, he can tell by its tone, but he can't hear over the agony thrumming against his senses. It's too much. He should be dead. He would be dead, but...
"-chei! KOSCHEI! We have to go, we have to leave, we can't stay here! Please, Koschei, get up, try to get up! I can't carry you...." He feels so far away, floating lifelessly beneath still water, insensible to the hysterical sobs reaching out to touch him, to force his eyes open, to pull him to the surface -
With hands covered in blood, Theta finally drags him to his feet. He tries to focus on what's in front of him, to focus on something besides the pain, but the other's face is so emotionally battered and devoid of its usual childish naivete that he doesn't recognize him, and he can't bring himself to look at him again.
They run forwards through the short blue-green grass, its floral scent putrid with the tang of charred earth, until they reach a copse of trees, where the sounds of battle are far enough away that they slow to a stumbling walk. Theta spies a lonely body rotting in the bright sun, and the thought of what it had been is more than he can take; he leans heavily against a tree, scratching himself on its many foot-long spines without noticing as he heaves and retches until he is exhausted. Feeling purged and achingly empty, he wipes his mouth clean with one shaking sleeve and, when Koschei looks away, wipes his eyes with the other. They continue on their way, refusing to say anything, while one of them hardens himself against the devastation, and the other runs ahead, eager to leave it all behind.
By the time they are in sight of the stolen TARDIS, danger has found them again, and without hesitation, Theta grabs his hand in a fierce grip and yells for him to run as they break from the poor cover of sparse, spiny trees, pulling him into a reckless sprint towards safety. An excited desperation pushes everything from their minds except for what's in front of them, but Koschei hears a faint, curious buzzing sound like small, hard-shelled insects flying past; it's only when they reach the door of the police box and Theta is fumbling with the key that he realizes they had just dodged a volley of darts.
There is movement in the corner of his eye, and he turns to find one of the native creatures looming threateningly over them, its multifaceted eyes unreadable.
"Theta, hurry up," he whispers urgently; his head is pounding painfully now that there is nowhere left to run.
Then, several things happen at once: the alien emits a noise meant to alert every bloodthirsty insectoid within range of their location, so he casts about for something, anything that he can use to shut it up - and grabs one of the incredibly sharp thorns from a nearby tree, determinedly bending it back and forth until it snaps off.
He acts quickly, before he can dwell on the idiocy of his actions, and rams the thorn with enough force to drive it through the beast's thin exoskeleton; and then he does it again, and again, and again, careful to avoid its flailing limbs while watching it with a kind of sick satisfaction that surprises him, but why should he feel guilty when that thing would have done much worse to them?
"Koschei!" He feels himself being pulled forcibly back by wiry arms wrapped around his middle. For a moment, he struggles against his restraints as if possessed, consumed by an unreasonable urge to hurt, wound, mar, kill because of the pain and fear that they had suffered, because these violent, primitive animals shouldn't be allowed to spawn, because it was so much fun.
Suddenly, the enraged creature lashes out and, being unable to move quickly enough, the blow knocks them both back through the open door of the TARDIS; Theta's grip on him slips and he falls heavily, smacking his already wounded head hard against the floor. His vision dims and the hateful wrath slowly seeps from him like heat through ice as he watches Theta slam the door on the approaching hiss of clicking mandibles and lock it before turning to face him...
I am so sorry.
Koschei wakes up, lying on a suspended platform in an empty, sterile room, back on Gallifrey. Sitting up, he runs a hand over the healed skin on back of his head in remembered pain, and when he sees no sign of Theta, he dresses hastily and hurries out of the medical bay unimpeded. His apprehension grows as muscle memory takes him down an often-walked path, and when he reaches his destination, he understands why.
Because the TARDIS is gone, and guards are waiting for him.
He is still hopeful while he is tried for the crimes that they had committed, but when he is executed, Theta doesn't return. Nor does he come for the next trial, or the next.
Years pass, and he is a different man, lying in a cell where he is kept when he isn't needed. Then, there come four knocks on his prison door - the sound of purpose.
So, he answers. And when the Master leaves, he doesn't come back.