As the episode explained, “It’s a TARDIS, remembered,” a collection of memories of the Doctor’s previous adventures that’s made manifest in the Time Window, and powered by the strength of memories: “If time is a memory, then memory is a time machine.” Unlike the Doctor’s actual TARDIS, it also doesn’t come with alien god of death Sutekh piggybacking on it, which makes it a handy way for the Doctor, Ruby and Mel to escape the wave of death Sutekh unleashes.
How long had Sutekh clung to the TARDIS?
“I have travelled with you for all this time, riding the spine of your ship, evolving into my true godhead.”
The way Sutekh tells it, it seems like he’s been riding the TARDIS ever since the Doctor trapped him in a time corridor to age him to death at the “Pyramids of Mars” (1975) climax. Instead of dying, Sutekh tells the Doctor that he found a home by clinging to his “infernal machine” and hiding “for so many years.” That doesn’t explain whether, when Fifteen cloned Fourteen’s TARDIS at the end of “The Giggle”, Sutekh was also piggybacking on that one.
But Sutekh is dead now?
Yes. Or as dead as things on Doctor Who ever are. The Doctor (with a conflicted conscience about it for some reason) cut him loose in the Time Vortex and he burned up.
What was the Susan Triad trap?
Everywhere the TARDIS landed while Sutekh was parasitically clinging to it, he used his psychic powers plus the TARDIS perception filter (the bit that makes it seem inconspicuous to passers-by) to create the same woman, multiple times over. The perception filter meant that each version of this woman fitted in contextually with her surroundings, hence her appearing as different species in different planets.
When Sutekh finally revealed himself and released the wave of death, each of the various Susans across the many planets acted as his conduit to spread it. “I birthed them all – my angels of death, now standing triumphant across the universe.” The Doctor felt responsible for the wave because it killed everywhere he’d stepped foot, like his carbon footprint catching up with him.
