Eight:
When the boy turns eight, he is taken from his home as all Time Lord children are. But instead of ceremony and initiation, there are tests of coordination, logic. A laboratory full of grim men and women. They tell him that he is special, different (he had guessed already). Made to save his people in war. That he must be brave. He is honored, impressed, in the little time he has to be anything. They connect him to a machine and tell him to close his eyes and stay very calm. Then, they feed him to the Master.
Nine:
Koschei forgets his ninth birthday. The transfer had failed, left his memory riddled with static like a bad radio transmission. There's almost a sense to the noise if he concentrates. But he's told (ordered) not to waste time. He's functional, if incomplete; perhaps after the war.
On the day, otherwise unremarked, he's given schematics for review. A new series of dreadnoughts, with the flagship Cruciform. His role is advisory: now that they have him, they don't quite know what to do with him. And it's another test, of sorts.
Then Skaro is destroyed, which most find cause for celebration. Some look slantways at the diligent boy sitting cross-legged in his chair, still at work. Wondering how soon they can put him back in a box. Koschei pretends not to notice.
Ten:
In his tenth year, Koschei breaks into the Matrix. It takes four tries to find himself and retrieve the memories that he'd known were missing. That they'd kept from him. His family. The Doctor. His name. He hides them all away behind the drumming noise.
Even so, he catches himself thinking of the Doctor at the wrong times. Imagining he'd find this place, and tear it apart. Or rather, lecture at them while almost-accidentally sending their laboratory into meltdown. It's the right kind of thought, a child's wish for divine intervention. Entirely wrong level of detail. And he'd never let the Doctor see him like this.
The Master goes to war that year, under his keepers' eyes. One of them the mouthpiece for his recommendations (orders). He wins them three battles outright, two over worthless colonies, ill-placed by the Council in its wisdom. He does far better in simulations, where he can dictate strategy and acceptable losses. The solution seems to only be obvious to him: he needs Gallifrey.
And then the Cruciform.
He remembers walking out while everyone still thought they'd won, just before the Cruciform's guns could swing around. He remembers typing coordinates into a Tardis without setting the manual override.
In between and for some time after, nothing. A blind terror that drowned out even his sense of time, even the drumming that lives in the back of his mind. From the Tardis's present course, he'd managed to communicate that much.
Half-way to the end of the universe, the Master can't think of any reason to go back. Not now, with the Time Lords losing the war decisively. Not like this, a half-grown boy.
Eleven:
At nine hundred years old and not quite eleven, the Master travels further in the future than any Time Lord in the record of the universe. So far out his species awareness has become directional, urging him back, back, back. It's unbelievably dark and empty. But there are humans, even now.
He's been human before. Twenty years with that sped-up metabolism will give him an adult body. But he is the Master. He's never lost himself, and all he has to do is remember to open a pocketwatch. In twenty years. And then he'll deal with Gallifrey, the Daleks, the Doctor, and anyone else on his list that's survived.
There's nothing on the Tardis that won't brand him as an alien to the humans, and a Time Lord to anyone who might follow this far. He hangs the pocketwatch around his neck and strips down. Sets the chameleon arch and stumbles through the open door, only another refugee.