Dahn has no idea when he was marked.
It was hard for him to notice it at first, because it's on the back of his neck, and normally covered by his collar. Yet a mirror will prove that a red marking is now there and refuses to leave by water, soap, or any other method of removal.
A straight slash, turning to the left at the bottom and ending there, while towards the top a curl to the right sprouts off of it. It doesn't resemble a letter or symbol in any language Dahn knows, and for a while, he considers passing it off as a scar from something he doesn't remember.
Demons treat him differently than before, giving him a wide berth. Even Taromaru is uneasy around him, and tends to skitter off if he doesn't have work to do. Dahn doesn't mind having demons leave him alone, but his own Taromaru not being able to be near him? He can't let that last long.
The smart thing to do is ask Nagi for help, so he does. She nods in understanding, then gasps when she turns around and lowers his collar.
"That bad?" he asks, without turning back to see her expression. He doesn't think he'd like it.
"H-hold on. Let me check my books to see if my conjecture is correct... I wouldn't wish to worry you with a faulty theory," she says. A mad shuffle of paper, and when he turns around she has her nose in a book, flipping furiously.
He wonders if he should have just went on not knowing.
She stops on one page and swallows harshly before beginning to explain. "Demons are commonly associated with sigils. These sigils can be used to summon them, but they are also used by the demons themselves: to mark territory, to claim lesser demons or humans as their own, and so on." She shows him the page.
The mark is reproduced in perfect accuracy. The top of the page is titled 'Lucifer'.
"Yeah. That's bad."
Dahn doesn't remember the rest of that conversation.
It is a week later, when he has almost made an attempt at putting the fact that he has goddamn Lucifer's mark on him out of his mind, that the blond asshole who made him start an apocalypse shows up, just casually walking up to him when he's in the woods. "You look well," Louis says.
Dahn punches him.
Or at least, Dahn tries to. Horrible pain suddenly flares up at back of his neck and he can't connect. "Bastard," he growls out instead. "You've got a lot of nerve, showing your face to me."
Louis looks surprised. "Really, Dahn. Is violence necessary? After all, I never lied to you. I only gave you what you thought you wanted."
Dahn only snarls.
"Very well. I can see that I'm not going to win you over." Louis sighs heavily and gently touches the back of Dahn's neck, where the pain instantly soothes. "Shall I simply take my leave already?"
"Wait." Dahn can put two and two together, and he'd already had his suspicions. "Why the hell did you mark me?"
"Oh, so you noticed?" Louis smiles. "I couldn't allow any other demon to take advantage of your impressive gullibility. Simply put... you are mine."