There is a space at the bottom of the world, where Earth and Ice and Shadow meet. It is cold, but not cold enough to kill; dark, but not too dark to see. A small round room, made of chilly black marble, lit by a dim and sourceless glow, with a spiral stair climbing the curve of the wall and a shallow circular recession in the exact center of the floor. The recession is maybe six feet wide by six inches deep, lined with something resembling pale frosted glass, and there is nothing in it.
"Several of them, if you need extras."
Here are the storage rooms in question. They have both of the advertised qualities: stone floors, full of things. (Charcoal is among the things.)
Cor did not have a lot of room in his pockets when he arrived but he did bring a sponge.
"The trust you have from these people is astonishing," Telarin observes when he catches up to Riale. "What did you do to inspire a man twice your age to be ready to make an uncertain journey to a dying world the moment you suggest it?"
"Things," Riale says vaguely. "Anyway, he's not twice my age, he's only thirty-one."
"Things," says Cor.
"I've led a busy life and Simm's known me for all of it, I don't actually know which things had that effect on him. If we'd seen Evai first and you'd asked the same question I would've said 'oh, I caught the guy who burned down her grandmother's house'."
"Well, I hope he is actually okay with bringing this letter to my world and not just vaguely obliged or something."
"One of the major criteria for which people I'm willing to ask in the first place is 'will they be actually okay and not just vaguely obliged'. It's - they know that when I take something seriously it's because the thing is serious, that when I say 'this guy needs somebody to volunteer as a messenger to his dying world to tell them evacuation is an option' those are the facts of the situation and if I'm asking them to do it then I must have thought it through and decided they're a good pick for the job, and they value my assessment highly because I know the imperial staff really well on both the organizational and the personal level and I'm smart and insightful and have good priorities."
"All right then."
"You sure about this?" Cor asks him.
"...okay. So, I speak this language for irreproducible reasons; you're not going to be able to talk to anybody at home. I will draw you a map - I'm not sure how exactly I can aim you but I can probably get close -" He draws a map. "You want to be here. Ask for Ranary; if they can't get you Ranary -" he lists more names. "- and give them this letter. I left blank whether or not they should send you back if that's feasible, should I put that you would like to be sent back?"
Cor fills that in. "This is the part where you get down to your underwear and I paint blood on you and draw on you with charcoal I'm sorry about that part."
"Cor's magic system has many inconvenient aspects and that is one of them."
Cor does arts. He gives Simm the letter and the map to tuck into his waistband. He does more art.
He chants (in his language, so the "I destroy, I destroy, I destroy" part is less ominous).
And then poof he is gone.
"I assume he notified all the right people that he was going away for an indefinite interval but I'm going to go tell Vira anyway just to be sure, I'll meet you back at the swoop."
"Somebody will send him back soon unless things are disintegrating back home much faster than seems likely," Cor says, and he heads back for the swoop. Then he turns back and voids all the blood and soot on the floor.