Harry's day had not gone well, and that evening he had relieved the agony with some therapeutic self-mutilation on the Astronomy Tower. The Bloody Baron had kept lookout for him, moaning in an approving tone once he saw what Harry was doing. A couple of couples had tried to get onto the roof to canoodle there, but the Baron had roared and waved his chains at them and they had run away screaming. Harry was intensely grateful about being left to hate everything in peace, and made a mental note to buy the Baron some Blood Lollipops next time he was in Honeydukes. With that in mind, he waved goodbye, was answered with a gurgle, and set off back to the Gryffindor dormitory.
In the dorm, all was quiet. Ron pretended to be asleep, knowing that a Potter post-mutilation was not a good conversational partner. Grateful once again, Harry unwrapped the towel from his arm and quietly muttered a Healing Charm at it, watching with regret as the lovely darkness vanished.
At that point Seamus rolled over in bed, groaning, and his right arm slipped out from under the covers; and Harry was astonished to see that Seamus bore a beautiful set of scars, which were unmistakably in the form of letters. As far as Harry had known up till then, the only self-mutilators in the castle were himself, Snape, Millicent Bulstrode and Moaning Myrtle, who some might argue didn't count, because she couldn't make any further holes in her ghostly body. However, Seamus's arm had unquestionably been sliced and diced.
And the words... Peering closer still, Harry read, "I Am A Wizard, Not A Baboon Brandishing A Stick."
Although Harry admired the sentiment, this seemed like a fairly unusual legend for a self-mutilator to choose. He himself usually carved "SB-CD-LE-JP", lots of little hearts, or random slashes designed to draw the most blood. He had occasionally branded himself "Idiot" or "Evil", and happened to know that Snape had a set of silvery letters the entire length of one leg that read "I Am A Worthless Piece Of Shit", but found it surprising that Seamus had chosen a sentence so positive and, frankly, long. It was most mysterious.
He sat for a while in quiet perplexity, then whispered, "Seamus?"
"Urgh!... Ur... Hi, Harry."
"Erm, you know that detention you did for Flitwick? He made you write lines?"
"Bloody hell, Harry, it could have waited till the morning, mate. I was having one of me dreams about Madam Pince."
"Yeah, sorry, but," Harry said doubtfully, "did Flitwick give you this... black quill?"
"Yeah!" said Seamus, eyes widening. "How did you know about that? He didn't give me it, I found it in his office, so I thought I was supposed to... Harry?... Harry?"