He had always wanted a bosomy wench dressed in Slytherin schoolgirl clothes. Not because he liked bosomy wenches (or even wenches of any sort), but because he knew it was so crass it would Piss Lucius Right Off. That had always been a good thing to aim for. Lucius was amusing when he was angry.
He wanted something that would be the antithesis of the Dark Mark. He thought for a long time, but the answer that kept popping into his head (a bunny, frustratingly enough) didn't seem logical. For several weeks after the Final Battle, he walked around asking people what they thought would be the opposite of the Dark Mark.
Bill: Erm...an open eye. Like the Eye of Horus. It's a symbol of protection.
Luna: I associate the Dark Mark with green and snakes, so I suppose a red-furred bunny would be a good opposite.
Harry: ... [just stared at Severus until Severus went away]
Minerva: The Hogwarts Crest, I think. Or a patch of tartan plaid. Or perhaps the flag of Scotland. (To which Severus responded, "I'm not asking what tattoo to get, you daft woman! I'm asking what is the opposite of the Dark Mark!")
Hermione: Ooh, are you thinking about studying psychology? This is just like a bit of the therapy they do. They give you something and you're supposed to say the first word that pops into your mouth. It's fascinating, the things people come up with. Here, let's try it! Dark Mark. Um, okay, what about Potion! Professor? Professor, why... Erm, I'll just go, shall I?
Remus: I hadn't really thought about it, but that's a fascinating question, Severus. I would have to say something that is full of life, something to symbolise rebirth, perhaps. A phoenix rising from the ashes? Why do you ask? Are you having nightmares, too?
Dumbledore's portrait: Why, love, my dear boy.
He thought about a dragon, twisting around his forearm, with a burst of flame coming out of its mouth, positioned right over the scar. Then someone mentioned Charlie Weasley, which reminded Severus that Charlie was into dragons. He thought that over and decided someone might get the wrong impression. He couldn't imagine who might, but Lupin was just daft enough that it would probably be him.
For a brief time, after Nymphadora Tonks' ridiculous outburst at Potter and Ginny's wedding, Severus considered a full moon, simply out of spite. After all, the moon had had a surprisingly huge effect on his life, so he thought he would be justified. But it would be, to Tonks, like Severus was staking a claim, and it would be, to Lupin, as though Severus were simply tormenting him about his Dark Creature status.
But Lupin pulled Nymphadora aside, and Severus just happened to overhear him delivering a dressing-down the likes of which Severus had never heard from the wishy-washy Gryffindor. There were harsh words such as "no right" and "no business" and "why can't I make you understand there is no future between us?"
Tonks had replied, "But if you'd only--"
"No," Lupin said sharply, cutting her off. "Tonks, I like men. Not women, and not women who can make their bodies look like a man's. I'm sorry, but you should have listened to me sooner. Now I'm going to have to make some explanations I don't look forward to."
"You wouldn't be bothered if Snape hadn't been here," Tonks groused.
Lupin's voice was dryly amused. "If you've noticed that, you should have noticed a lot sooner that I'm gay."
He had it narrowed down to a phoenix rising from the ashes (and so what if it was Lupin's suggestion, it was a good one) or a potions bottle next to an expensive potions ingredient (and so what if it happened to be something necessary for the Wolfsbane Potion), when Lupin caught wind of why, exactly, Severus had been asking questions.
"You're getting a tattoo to cover the scar?" Lupin frowned at him. "Why?"
Severus stared at him as if he were daft, which Severus wasn't always convinced wasn't true. "That scar represents everything I hated about my life. Pardon me if I want no reminders."
Lupin's fingers were suddenly resting, ever-so-lightly, on the dark, slightly depressed skin that had been the Mark. "That isn't how I see it," he said softly. "To me, this scar is a mark of your strength. You survived the war, you came out triumphant. You did what you set out to do, and you are a stronger man because of it."
There was a moment of silence. Then Lupin spoke again.
"I find that incredibly sexy."
And One Tattoo Severus did get:
SS ♥ RL
In bright red ink but very tiny, straddling the spine, just above his arse.
He thinks of freedom every time Lupin's lips ghost across his tattoo.