A/N: I can't say this idea is soley mine. Dear Lunacy and I thought of this some time ago together and I am soon to miss her terribly. Also, I felt it explains the mystery of Magnus's prestige.
"No, I'm sorry," Magnus answered. "The most magnificent me cannot remain here tonight, My Dear Alec," He sighed before adding; "Not that I'd mind spending the night alone with you again... Alas! I have suppliants who wail for my attention!"
Alec blushed slightly and chose to ignore his boyfriend's comment, for the sake of his own sanity. Not, of course that he and Magnus had never done things he'd lie to his Mother about, yet still, he was not one to be entirely comfortable. "That's alright." He shook off the embarrassment. They were alone, in any case, and it wasn't like Chairman Meow was going to spread the world. "High Warlock stuff?"
Magnus snickered a bit, and raised an eyebrow at his boyfriend's genuinely confused expression. "Oh, you were serious, weren't you?"
"Er... yes?"
"Darling, let me ask you this. Where's the High Warlock of Manhattan?" A bemused smile still graced Magnus' painted lips while Alec seemed to be wracking his brain.
Surely he should know this! He'd attended clave meetings, he took to memorizing Lillith's names! (And by the Angel, she had a lot.) Alec cursed himself for his evident ignorance. And Magnus did bring up quite the point. Why had they always gone all the way to Brooklyn when in need of a Warlock. The Institute was, indeed, in Manhattan. "I'm sorry... I don't know."
Magnus put an arm on his confused lover's shoulder and shook his head. "That's because there's not one."
"But then... why are you only the High Warlock of... Brooklyn? Not New York?"
Magnus sighed theatrically. "Because, Dearest. It's a false title."
"What?"
"Why, Alexander, would there be a High Warlock for just Brooklyn? I made the title up forty years ago to see if anyone would believe me and you're all so comically gullible I kept it up! Not that I'm not flattered..."
Alec, seeing the utter sense in all of this suddenly felt quite silly. And a bit sad that no Shadowhunter had ever realized this before. Maybe they didn't know everything about the Downworlder community after all. "... Oh."
"Just don't tell your little friends," Magnus pecked him on the cheek. "I want to see how long it takes them to get it."