[Insert Disclaimer Here: I don't own the characters. In fact, I barely own the idea. I wrote this for an anonymous prompt on Naruto_Meme and thought it was worth sharing.]
Warning - This fic authentically depicts a more human equivalent of shark mating rituals. That means it is violent and sexual.
Itachi groaned as he climbed into the shower. He stood under the scalding water, letting it rinse four days worth of blood and grime from his battle-weary frame. Their last mission had been an exhausting one - too much fighting, too much bloodshed, too much death...
"No matter how many times you shower, you're never clean, Itachi." Kisame grinned, leaning against the bathroom wall. "I can smell the blood on your hands. Just as clearly as I did six years ago. It doesn't go away. It never goes away."
Itachi ignored him at first, throwing his head under the tap. He ran his fingers through his tangled hair, tearing at the knots until they finally unraveled. Kisame, however, wasn't in the mood to wait.
He slid out of his cloak, throwing it over the door next to his partner's. He shed the rest of his clothes, tattered and filthy as they were, across the bathroom floor. He paused for a moment as he untied his headband. Itachi's clothes, which fared no better than his, were folded neatly in a pile. His headband and all of his jewelry, even his coveted Akatsuki ring, sat on top of them. Behind the curtain and underneath the water... his partner was completely naked. He wasn't Akatsuki. He wasn't a shinobi. He was just a man - a vulnerable man.
"I wouldn't want it to ever go away..." Kisame licked his lips and pulled the curtain back. "I like it when you smell like that."
He climbed into the shower, seizing his partner by the upper arms. Itachi threw his wet hair out of his face, but offered no real resistance. Kisame didn't expect that he would. It was moments like this, when they had stripped off their allegiances and their obligations, that the tables turned. Kisame became the predator and Itachi the prey, and in the heat of the moment, neither ever questioned it.
Kisame growled, aggressively pulling his partner closer. His black fingernails dug into Itachi's skin, drawing more of the blood he so fiercely craved.
"You always have..." Itachi leaned his head back against Kisame's chest, exposing his neck like a wounded animal. The shark inside Kisame throbbed, watching the knob in Itachi's throat tremble as he spoke. It was mesmerizing. Blood ran down Itachi's arms in small trickles and it was intoxicating. He couldn't resist it, or his partner, any longer...
He bore his pointed teeth and drove them as hard and as fast as he could into Itachi's shoulder, right at the base of his neck. Itachi thrashed in the water, and Kisame tightened his grip. He held him, tearing through layers of flesh without hesitation or remorse until the thrashing stopped and breathless screams turned into painful moans. Blood dribbled down his chin and ran down his throat. The water at their feet was burnt orange. The basin would probably never come clean. And Itachi...
Jets of water were like daggers against marred flesh and open wounds, but red eyes flickered with morbid desperation. His lips trembled. For a brief, fleeting moment, Kisame thought he may have gone too far. But Itachi was hardly as delicate as he seemed on the surface. He was groaning with pleasure, not pain.
Kisame swallowed a mouthful of blood and bit into him again. This time, Itachi did scream. Sharp teeth gnawed at his shoulder and grazed down his right arm, scraping his thick skin raw in a single sweep. Nails dug into his chest, his sides, his back, tearing open old scars and creating new ones. When, and only when, Itachi's mutilated body threatened to pass out beneath him, Kisame spread his legs and drove himself into his partner's waiting ass.
It was hot. It was heavy. It was unbelievably violent. Water sprayed over their faces like a torrential current. Kisame relished it. Itachi struggled to breathe and it wasn't long before his legs finally gave out. Kisame sank to his knees, continuing to thrust, unrelenting, against his partner for another ten minutes.
In a sudden shudder, the sadistic orgy of agony and ecstasy was over. Kisame tore himself free from his partner and climbed out of the tub. Itachi collapsed in the remaining pool of bloody water. He was literally so full of Kisame's seed that much of it sprayed out into tub in a ropelike mass. He whimpered. He coughed. Kisame laughed.
He stood over the tub, arms crossed, waiting for Itachi to finally lift his head. Black eyes, blurry and glazed over with pain, tenuously met his. Itachi pursed his lips, but no sound came out. The intent, though, was unmistakable.
'... More.' he mouthed, just as he always did. Kisame shook his head.
"Soon." he promised, turning away. He shook his hair dry, and let the rest of him drip onto the rugs below. Floorboards creaked as Kisame sauntered into the bedroom and climbed under the blankets. The scent of blood was so thick that it permeated the entire room. It was like a lullaby, and he was more than ready now to succumb to exhaustion.
In the distance, he could vaguely make out the sound of his partner crawling out of the shower and collapsing on the hard, tile floor. To some, it might have seemed cold, if not downright cruel, to leave him that way. But once the tap was turned off, Itachi wasn't a man anymore. He was an exiled shinobi, a cold-hearted murderer and a proud Uchiha. And Kisame knew, without a doubt, that he'd wake up to find his partner sleeping peacefully by his side, pleasantly oblivious to the blood-stained sheets beneath him.