He was old beyond imagining. He had lived through time, walked up and down and to and fro in it, so many times that he was actually older than the universe itself. But, he remembered it all.
His own past was a vast, rich tapestry to him: so many faces and places were woven into it, were its color and texture, that he could lose himself to pondering his own memories for days at a time. Appearing unconscious, he was alert and alive, reliving moments that crossed and re-crossed the millennia, feeling each moment again.
Certain faces appeared again and again and again.... The Doctor was one, of course. How much time had he spent with the Doctor? More cumulative hours than any normal human had lived? Longer than Earth's sun had shone? The Doctor had been with him since his own beginning, was in part the cause of his beginning.
Back then, he had had a complete body, had walked and talked as other men. He had laughed and loved, cried and died. Died? Oh, uncounted millions of times. He had died more times than he could be bothered to count by the end of his first thousand years. And even that was so many personal millions of years ago that he couldn't be bothered to count them. He had experienced every death imaginable, most of them many times over.
Every death had cost him: the pain, the darkness, the fear. But, even these faded with time. Some had cost him even more: irrevocably destroyed atoms that then failed to return to him when he arose. Death in the heart of a star or by anti-matter will annihilate even hydrogen, that most stable of atoms. Over the billions of years, those losses had added up. His very substance became thinner, grew attenuated.
He wasn't the first to notice the changes. At that point in his personal timeline, he had been working with a medical team, fighting a plague on some backwater world, tending the dying and looking for a cure. A man who returns from the dead has many different careers and this one had been challenging and fulfilling. As a matter of course, the team analyzed each other's health microscopically every day, fearing exposure to the contagion. Matching scans to previous records, they were able to observe any changes as they happened.
He'd gone off-planet to acquire supplies, run into some difficulties, and sent a "mayday" back to base as his ship plunged into a nearby star. His team, aware of his strange ability, sent another ship to await his return. Their scan of him that day failed to match his previous scans: Something was missing. It was just a couple of cells, a few thousand atoms, but they failed to materialize.
It was then he discovered that maybe--just maybe--he could actually die.
As the eons passed, he had kept a close watch on his physical frame, tracking the gradual decrease in his substance. A man who has no fear of death has many adventures and he had been a stellar example of this. Even knowing that he was gradually diminishing, he couldn't stop living and sequester himself, retire to a safe haven for eternity. So, more atoms disappeared. And as they disappeared, the remaining atoms, on returning, rebuilt the most necessary parts of his body first, leaving out inessentials. His body became simpler: fewer toes, then fewer fingers, and then fewer limbs. He had had to use prostheses, then each century's equivalent of the wheelchair. Finally, his body reduced to a vast head spread almost vaporously thin, he retired to a support tank that supplied his few remaining physical needs.
As his physical capacities diminished, however, he discovered an unexpected side effect: his mental capacities increased. First, his memory became even more precise. Then, his sensitivity to other people's emotional states grew. Eons later, he developed the ability to project his own emotional state onto others. Next, he could directly alter others' emotional states, regardless of his own. And finally, he developed the ability to project his very thoughts into the minds of others, obviating the need to speak using his physical body at all.
Living forever and traveling the universe as freely as he had, he was able to take advantage of long-term investments everywhere. He'd spent most of his existence a very rich man although he'd lived very simply for the most part. And, as he'd jumped back and forth through time so much, it had been easy to create a new persona whenever he wanted. Thus it was that the legend of the Face of Boe arose. Already impossibly old when he retired to his support tank in a time not too long after his original birth, the Face of Boe was known throughout the rest of time as a several-billion-year-old entity with an unparalleled knowledge of universal history. No one, not even the Doctor, had ever made the connection between this bottled face and any of his walking, talking personae. It was the ultimate disguise.
And, now, at the end of it all, he waited on this "New Earth" for one last visit from the Doctor. Attended by Novice Hame, spending his vast energies keeping New Earth's people alive, he knew he was fast approaching his final death, his true release. Death and the Doctor, two friends he had carried with him through all of time were coming together.
He delivered his final message to the Doctor and embraced Death. And, as his consciousness receded into the infinite one last time (calmly, serenely, without fear), he contemplated that vast tapestry that had been his life. The warp and woof of the tapestry held such detail: the faces of thousands of lovers, of billions of friends. As he receded further from it, the great collage gradually changed into a single image, the many faces composing a single face with sparkling eyes, the eyes of the greatest and most tragic love of his unimaginably long existence: Ianto Jones.