vampire batman who has made a point to never, ever feed on people and will starve himself before doing so if he can't get his hands on any blood.
and one day superman is in trouble. he's been tied up and tortured at knifepoint. batman infiltrates the warehouse and takes out all the hired muscle, his teeth aching with this much adrenaline-infused blood so close. he forges on to the room where superman is being kept, finds him strung up against the wall, his wounds already healing.
bruce has already incapacitated the doctor who'd been hurting superman, so his only objective is to free him and escape.
he moves closer. superman's face blooms with relief, but before batman reaches him, something on the table catches his eye.
he stops dead. there, on the table, lies a long knife, gleaming with superman's blood.
batman knows he's doomed the moment he spots it. he's been starving himself for days already, and that blood is right there. it's not—it doesn't count if blood is already—but no, it's kal's blood. blood on a knife he's been tortured with. bruce couldn't possibly—
a sharp pang of pure agony lances through his teeth. batman has the knife in his hands before he can think to stop himself. he brings it to his lips, saliva pooling on his tongue. he knows it's wrong, knows he should untie kal and not eat his blood in front of him, but he's so, so hungry, and the blood is here, in his hand, millimetres away from his mouth—
he darts his tongue out, lets it catch against the flat of the blade. kryptonian blood is sweet, sweeter than any batman has tasted before, smooth and filling and perfect. he licks the knife again. and again. he keeps his movement deliberate, delicate—careful not to nick his tongue on the sharp edge.
superman—still stuck against the wall, helpless to do anything but watch—can only stare, wide-eyed, as batman's perfect pink tongue touches the blood, his blood, and comes away red before disappearing back into his mouth. clark can smell the satisfaction on him, can see how wet his tongue is even before licking the knife.
superman's heart pounds against his ribs. he'd do anything to feel those teeth against his neck; he's already shivering at the thought of that rough tongue trailing along his pulse point. batman must be hungry—it's the only explanation for his behaviour, deviating from the mission—and superman can fix that. he can be a food source for batman, without any worry of injury or long-term effects because of his healing. it'd be perfect.
he watches batman come back to himself in real time. batman stills, the knife halfway to his mouth. his eyes flicker up to meet superman's. his pupils are dilated. slowly, so slowly, he returns the knife to the table, moving silently towards superman and deftly undoing his binds.
"i—apologize," he bites out. superman can smell the metallic tang on his breath. "i should not have done that."
superman can hear the regret in his voice, can see batman pulling away from him, building invisible walls in record time.
"don't do that," superman begs, reaching for him. batman jerks under his hand, but doesn't move away. "it's all right. how long has it been since you've eaten?"
batman shrugs, small enough it doesn't dislodge kal's hand. "i've been without food for longer," he says, his voice quiet with self-deprecation.
"that's not an answer," superman insists. he wets his lips and tries a different tack. "if—if you need a reliable...supply, i could—i'll be that for you."
batman's jaw goes slack in surprise. "i don't—i would never feed off people—"
"i know," superman cuts in. "i know you wouldn't. but i'm asking—" kal bites his lip, tries again, "i'm offering. i know you won't hurt me. and," he pauses, glancing over to the now-clean knife. batman tenses beneath his hand. "it tasted—good. didn't it?"
god, how desperate does he sound, how obvious—
"yes," batman breathes. "yes, it did."