"It's good to breathe the air once more..."
He has breathed the air on every habitable planet and walked the surface of some that would smother him, back when he still had legs to walk with. Every time he has died, he has gone down into the darkness, wondering if this death would prove the last, the time when his luck ran out, or the curse would lift, depending on how he felt about it that day, about his peculiar condition. There is that strange and terrible period, the so-called "Miracle", when it seemed the universe might have released him from that state, but at a price he could not allow humanity to pay, yet that proved a temporary state.
But now, while he lies amidst the wreckage of the containment pod for his present form and amidst the cobwebbed ruins of a city nearly caved in on itself, he can feel the last sparks of life fading, while he gazes on his last three comrades: the Catkind female who, as penance for her involvement in a terrible mode of healing, had taken on the duty of tending his aging substance; the lovely lady with the voice of a Nightingale who had inspired Shakespeare to write of his "dark lady"; and the Time Lord who had gotten him into that state of near-immortality that had kept him in the universe for so long but who had in another form tried to find a cure for it.
Five billion years. Nothing to sniff at. It has been a good life, though the last two billion years have found his capabilities somewhat limited, in some ways. But now the end of the road has come, the end of his wanderings, the end of his existing. The edges of his vision have started to grey out as he feels himself starting to sink into the darkness. The warm, dry air, so soft on his weathered face has started to cool. But he can still look into the face of the Doctor, that one young face of which he has felt so fond, since the time when they had both been young and full of fire. But his fire has faded and his time has come: a gentle numbness has started to creep over him, like rising waters around a ship in a Viking's funeral. One might call it a nothingness, a non-feeling, but his emotions have not worked as a human's would, since the horror that had mangled him into his present form. Yet after ages of feeling so many things -- pain, sorrow, loss, occasional joy -- it feels good to feel a release from all that, to feel himself slipping away as fate or the universe releases him from its grip. He has not felt alone in the universe for quite some time, not since he lost everything below his jaw, but he can feel the Doctor's loneliness, can feel the loss that his three comrades exude. he can see that the Doctor, in this form, has not yet experienced some of the events that he has. And so he gives them a hint of things to come, couched in a reassurance:
"You... Are... Not... Alone."
And then he can leave the universe. Then he can leave behind the father of the Children of Time. He has left his final blessing, now he can fade into the fabric of time and space...