He knows there's someone else there. Behind him, just where he can't see.
It's a feeling that's been following Altair for months now, an itching on the back of his neck, the pressure of someone watching his kills when he knows there's no one there. It irks him. He is an assassin- we work in the dark to serve the light- and there should be no one hiding in the dark to watch him.
But there is.
A kill is a private moment. The knowledge of what must happen fighting with the feeling (it never really goes away) that this is a man who woke up this morning and didn't know he had to die. No matter the man. No matter what circumstances conspire to bring Altair's blade to his throat. It never feels quite right, and Altair prefers to do it alone. Just him and the dead man.
The watcher isn't there all the time, which should be a relief. But it's not, because whenever he's not being watched from the outside, he can feel someone else looking out through his eyes, filling every space in his body, sharing his thoughts and his feelings, and this is an enemy Altair knows he can never fight, because he's starting to believe that the enemy is himself. His mind has betrayed him, and this is the beginning of the end. He has seen assassins go insane before. He always thought them weak.
For a long time, he waits. For his mind to snap, for the end to come. But when nothing does, he relaxes a little. There's someone watching him, from just behind him, where he can't see. From inside his head. And he finds that he doesn't much mind. Sometimes, when his mind is his own and the watcher is outside, he catches himself feeling lonely. He grows to know the mind that sometimes shares his body. He knows when the other is excited, or afraid, or annoyed. He knows these feelings as well as he knows his own.
When he races over the rooftops of Jerusalem, running from guards or searching for a target, he feels the watcher's joy, the same pleasure Altair once felt for the same exact reason- the freedom of the chase, of knowing he's too fast for the guards to catch, too clever, too skilled. And after that, he spends a little more time than he needs to on the roofs, climbs a little higher than he used to. And he knows the other can feel him as well. He used to feel horror from the other when he watched Altair kill. But he has to do it, even if he does regret it a little, and he tries to share that with the other. This he does for duty, not for pleasure. And after that he feels hesitation from the other, but never again shock or horror.
All these thoughts flash through his mind on the day he kills Al Mualim, and sees the golden globe shining in the air above him. The watcher is there again, and for just a moment Altair thinks of ignoring him. He has other problems now, other sorrows. But something makes him turn, and in that moment, in the golden glow from above, he sees the other for the first time.
His first thought is that he is looking at himself. The features are nearly the same, down to the scar just under his mouth. But the other's skin is paler, his clothes strange, his posture tense. He's staring up at the globe, confusion and defeat mixed on his face- and Altair stares at him in turn, trying to wrap his mind around the similarities, about the other's presence here at all.
He takes a step forward- the other's eyes snap down to Altair, his eyes widening, and Altair feels the other's surprise clear in his own mind- and then he vanishes.
Well this is embarrassingly short, but I'm in the mood for short. So sue me.