Author's notes: Joker/Batman from "Injustice For All", Episodes 1x18 and 1x19 of Justice League.
The title refers to the unwritten rules followed by villains, and the special understanding between those who know what it means to have an arch enemy.
As our ficclet begins, Luthor is sending up three of his henchmen to plant a bomb in Justice League headquarters.
Code of conduct and possession
"Remember, don't press the button until you know-"
"Yes, yes, take out the Martian first. Whatever." Star Sapphire didn't have any patience for the Ultra-Humanite's instructions.
While she, Grundy and the Shade got ready to leave, the Joker sidled up to Lex Luthor, who was tapping into a satellite that would allow them to watch as the Watchtower was blown up. "You know, Lexy, I was wondering… when the mighty Stupid League is floating through space in little bits and pieces…what happens to the one downstairs?" He seemed innocently curious, but there was something hidden in his voice, something sinister.
The businessman didn't bother to look up from the keyboard, "I have plans for him, so don't even think about it."
If he had bothered to look, he might have been a little concerned at the flash of pure hate that crossed the Joker's usually smiling face. "I asked nicely. You're about to have yours, so why can't I have mine? You think I came here for your charming company?"
The rest of the Injustice League had quieted down and was watching them. Luthor turned around and pursed his lips, considering the other villain, "I don't pretend to know what goes on in your mind, clown, and I don't see how I can owe you anything."
And before their very eyes, the Joker was transformed. From the annoying comedian bloomed something inhuman, something utterly devoid of reason, empathy, and mercy. In his eyes the Injustice League could see themselves cut open, mangled, bloody, and the echo of his laughter rang in their ears. Luthor alone seemed unfazed, though guarded.
The clown slid forward, hunched slightly but completely graceful, until he was right in front of the other man. "Lexy, Lexy." He reached out but didn't touch, "I want him. He is mine. Surely you can understand." His voice was dark and cut like a razor. The businessman raised an eyebrow. Joker's eyes grew large and bright. "Mine," he hissed.
Lex Luthor turned away, and for a second the rest of the Injustice League held their breaths, convinced that the clown was going to attack him with claws and teeth, but then their leader spoke, quite calmly. "Fine. When the rest of the League is out of the picture, you can have him."
And just like that, the monster was gone. Joker began to bounce up and down with his hands in the air, "Yay! Thank you, Lexy! Oh fun, oh fun, oh fun!" He sounded like a child who had been promised a shiny new toy, and laughed in delight as he pirouetted towards the door. The other villains breathed freely again.