“A stele is eight feet tall, covered in the dead languages by special Fifth adepts, and continually bathed in oxygenated blood,” said Mercymorn from the pilot’s seat. “The type of thing where, if there is one on board, you say quite soon, ‘Oh, look, a stele!’”

“Thank you, elder sister, I so love to be educated,” said Ianthe.

“Where we are going there are no obelisks for a stele to hook on to,” said the Emperor, whose fingers had ceased drumming on the back of the pilot’s seat (after his Lyctor had told her God, repressively, “That’s quite annoying, thank you,”) and were now restlessly adjusting one shabby shirtsleeve. “I am taking you both through the River.”

There must have been no small measure of blank incomprehension on your faces. He said, a little abstracted now: “It’s the only way. Faster-than-light travel turned out to be a snare—the way it was originally cracked, anyway. The first method destroyed something to do with time and distance, rendering it unusable for any good purpose…”

“I’ve always thought it should be correctly managed with wormholes,” said the Saint of Joy, doing something obtuse with the controls, “or spatial dilation.”

God said, “It’s in that wheelhouse. We came up with the stele instead, and the obelisk, which are less to do with travel than they are to do with transmission. But there will be times in your future when you will have to move unfettered by needing an obelisk, and even times yet to come when you will fulfil the sacred Lyctoral duty of setting obelisks, and that means travel through the River. I like to think of it as descending into a well.”

There was a small noise of upset from the pilot’s seat. “Teacher,” said Mercy, “it is the River. There is a perfectly good water metaphor waiting for you.”

“Well, I want the idea of two depths, and I don’t want to confuse them with the idea of speed where none—”

“—it’s the River, which perfectly well lets you say, Imagine the River—”

“Mercy, either you don’t like my previous, perfectly good river phrasing, or you do. Pick one.”

“I will not help you to make hyperpotamous travel happen, thank you for the option, my lord,” said Mercy.

“In that case, despite hyperpotamous being a perfectly good word that both catches the ear and does what it says on the tin, let’s deviate,” said the Lord of the Nine Houses, who apparently existed within a complex power dynamic. “I’ll use Cassiopeia’s.” (“Oh, no, the lava,” said Mercy.) “Girls, imagine a rocky planet with a magma core beneath the mantle.

“Travelling overland from point to point might take a year. If you understood your journey and the relative spaces well enough, you could instead drop into the magma, which would carry you to your destination in an hour.”