"Its complicated." Susan shook her head at the memory. "Grandfather had a disagreement with the Auditors and got fired. Before New Death took over, there was Death of This and Death of That manifested. Death of Mayflies was horribly rushed. When Grandfather took his job back, he absorbed everything else, but kept Death of Rats on to deal with the Rat Clan at Unseen University. Death of Rats has decided I'm amusing and follows me around. I'm surprised he isn't here."
"Oh, Ronny Soak - Kaos. Kaos got bored when the Horsemen broke up so he created a humanish aspect. Ronny's the milkman for Ankh-Morpork."
Susan couldn't help but grin. "You know how when people wake up in the morning with a hangover and say, 'Oh god, do I have a hangover'? That god's name is Bilious. He is one of the Small Gods, but there you go."
"In Grandfather's library is a wing where everybody's Book is kept, their biography, so Death knows where to find them. In another wing there is an hourglass for everyone alive so he will know when their time is about to run out. Of course it is just a manifestation of how people believe things work. Grandfather really keeps the list in his head, and I have inherited his abilities. Your Doctor is not on the list. I could tell he is not human, of course, but he is either immortal, or part of a different system entirely."
Susan took the time to study Donna for a moment. "You are human, but also not on my List, so I assume the latter."
"What are you going to do when you find this Doctor? He seems just as stuck here as I am."