Disclaimer: I don't own Doctor Who, Sandman, or any of their characters.
Against the Dying of the Light
The TARDIS sees her.
The heart of the TARDIS perceives all of space and all of time. There's no way it could avoid seeing her, even if it wanted to.
Sometimes the TARDIS almost gets the impression that she's following the Doctor. Wherever he goes, she's there. The TARDIS has learned to recognize her despite the varying disguises she wears. Her dark hair might be short and rumpled, or long and lustrous. She might be wearing a tank top and jeans, or a ballgown, or a kimono. The silver symbol around her neck (an ankh, its extensive library informs it, a symbol of eternal life originating in Earth's Egyptian culture) might be simple and unadorned, or embellished with curlicues and gems. But the general pattern is always the same, and the TARDIS always sees her.
She's there at the end of the Time War, when uncounted millions of Time Lords and Daleks go into the dark. She's there when Julius Caesar is stabbed to death on the steps of the Roman Senate, when Jack the Ripper cuts a London streetwalker to pieces, when a yet-unnamed baby in a tiny Third World country dies a few seconds after its birth. She's there on Mars, at The Library, on Alfava Metraxis. Time and place seem to matter as little to her as they do to the Doctor.
The TARDIS has files on all the life-forms it knows of, which is pretty much all of them. They are all cross-referenced into various subdirectories: by location, by the time period during which their civilization existed, by classification according to any number of different phylogenies. And by whether they are allies, neutrals, or enemies.
The sub-directory of enemies includes beings that all the starfaring races fear: the Daleks, the Cybermen, the rogue Time Lord known as the Master. The TARDIS has ranked them according to their danger, which it defines as their potential to kill the Doctor in such a way that he can't regenerate.
The Daleks are not at the top of this list.
Nor are the Cybermen.
Nor is the Master (which would surely enrage him, if he knew about it).
The individual whom the TARDIS judges to be the greatest danger is the woman.
It knows that one day soon, it will take the Doctor somewhere and see her waiting there. Not for the indigenous people of whatever planet they're on, or the people the Doctor's fighting, or even for an ally or friend of the Doctor. She will be there for the Doctor himself. One day soon, for the first time, the Doctor will see the woman that the TARDIS has always been able to see.
The TARDIS knows the Doctor, so it knows that he won't show any fear. He'll smile at her, and say, "Hello, I'm the Doctor. You'd be Death, then, I suppose?" She'll return the smile and admire his sonic screwdriver.
And then the Doctor will ask her a question, the question. The oldest question in existence, and the last one the Doctor will ever ask.
"So, what happens now?"
Of course, the TARDIS also remembers when the woman came for Jack Harkness. It remembers the experience of being inside Rose Tyler, and it remembers (in a way that Rose couldn't when she was only human once more) the expression on the woman's face when Jack began to breathe again. Her eyes had widened in surprise, and then she had smiled slightly and walked away.
The TARDIS knows it can best her. It's done so once before, after all. Perhaps it can do so again. Not directly this time, because the chance that one of the Doctor's companions (they're still his companions, even if they don't know that yet) will take its core into themselves is nearly zero. But it sees all of time and all of space, and it can subtly manipulate the course of events. It can nudge a wheel just a centimeter this way, short out a particular tiny circuit, and thereby send itself to a slightly different time or place than the one the Doctor intended. Not to mention the times when he spins the wheels and dials completely at random and lets it choose the destinaton for itself. And maybe, just maybe, by tweaking the right circumstances in the right way...
They're on a new world now, in a new time. As always, the woman is there. And as always, circuits (which are really just silicon neurons) fire, and lights flash, and alarms beep, and all these things form a thought.
I see you.
A/N: I've been rereading Sandman recently, and I couldn't help imagining Death meeting the Doctor. And then I got to thinking about the TARDIS as a sentient being, and the fact that we've seen it thwart death (or Death, if you prefer) once before, and how it might react to knowing that the Doctor's going to die.
The title of this fic is from the poem "Do Not Go Gentle Into that Good Night," by Dylan Thomas (which I don't own either).