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“You must let me mark you.”
Thor’s gaze is hard and serious. Loki perfected his shape-shifting spell some months ago and Thor has been sleepless since, imagining all the ways he could vanish.
“No one else will know. But I need something to be sure, Loki.”
A dozen protests pass Loki’s lips, and all go unheeded. In the end Loki (young, foolish, grudgingly flattered by Thor’s insistence) gives in to his brother’s bright blue eyes. Mjolnir hums warm and caring against the sole of his foot, and Thor’s hand is even hotter at the top of it. He cries out when the hammer burns him; eating skin and muscle and bone, a pain that shoots right through his leg and churns like lava in his stomach. When Thor is done Loki’s eyes are streaming, and Mjolnir drops to the floor trailing sticky drops of burnt blood from an incision on Loki’s heel: a goat’s horn, the root feeding into the end. Later (years later) Loki imagines he felt shame (for his arousal) and regret (for his compliance) but at the time he felt no such things.
—-
—-
Powder doesn’t touch it. Cream cannot cover it. Changing shape has no effect - Loki becomes a fly and his strongest right insect leg still burns. He becomes a mountain and Thor could still find him, if he dug in just the right spot. He fights as a woman, a giant, a venom-drenched serpent - Thor turns him on his back and his palm finds the mark. There you are, Loki.
When he cuts the damn thing off, it grows back marked.
—-
“I’ll be dead soon,” Loki croaks, as Thor digs savage, loving teeth into his heel, nuzzles the foot with his grey-bearded cheek. “And I’ll be reborn without your damned brand.”
“Not if I can help it,” Thor snarls, and pins Loki to the fur with his hands, his teeth, his eyes.