The amount of time Sephiroth and Cloud need to spend in the mako tanks is known. Predictable. It happens on a schedule, and follows a statistically-determined enhancement trajectory. What this means, practically, is that Cloud knows exactly when Sephiroth is about to pin him down and lock him in his worst nightmare.
It starts with a spar, every time. Even when Cloud is tiny and ineffective, it still starts with a spar. Sephiroth is amused. Cloud is determined and terrified, armed with the deadliest weapon he can get his hands on at that moment. When the battle begins, Cloud means it. Not that he doesn't, usually, but this is different.
Sephiroth praises him, corrects him, scolds him for being 'so terribly stubborn.' It's for your own good, my Cloud, do you not want to be strong? Very well, we can start with this. The darkness in Cloud's mind circles him, encompassing: a threat , although what exactly it's threatening Cloud refuses to understand.
They fight —
No. Cloud fights. Sephiroth shapes him, forming a mold of his own choosing with each strike of the blade. Resplendent strength. Godhood. Worthiness to stand at his side. And Cloud plays into it because he simply has no other choice.
He fights until he can't get up any more, until he's gasping for breath and drenched in sweat as he lays crumpled on the floor. Only then does Sephiroth gather him up in his gangly child's limbs and carry him to the tank. His tormentor is not shy with praise, and he is not shy with his affection, or what passes for it. The darkness is warm and soothing. Proud. My Perfect Tempest, so strong and determined, you will shine like the radiant moon when you finally understand where you belong. My side is the only place for you.
Every gem needs cutting and polishing to become valuable. Sephiroth is not shy about that either.
He gives Cloud the injections, easily pinning him to the table with one hand. He hooks up the monitors and the breathing mask, places Cloud on the cold, sharp metal grating of the tank's bottom. Fear not, says he, every action a mockery of tenderness, for I will make you strong again.
The door closes and the alarm sounds and mako starts to fill the tank . That's when Cloud, blinded by terror, finds the strength to get up and throw himself against the unbreakable glass of his prison. He screams, wild and unashamed, clawing at the walls until his fingernails tear, until the mild paralytic kicks in and he floats limp in a sea of burning green.
He only made the mistake of clawing at the equipment on him once. It backfired immediately as mako flooded his lungs, searing all the way down to the level of his cells. Technically it could never kill him, but direct contact with sensitive tissue meant he'd thrown off the mako absorption calculations. That was Sephiroth's concern, and the reason Cloud had been pulled quickly from the tank.
Cloud's concern had more to do with the partial dissolution of his consciousness into the Lifestream.
It had backfired in less immediate ways too. Cloud had gone back into the tank, of course, this time with a high dose of paralytic and Sephiroth himself. They were still children —a tank sized for a fully-grown man fit both of them easily. With Sephiroth so close, and Cloud so scattered, the warm, beckoning darkness had been almost too great a temptation to resist.
Almost.
He refuses to make that mistake again, even when blinded by fear and pain. So he claws at the tank's walls, and does what damage he can, until he has no choice but to float, still and quiet. Sometimes he watches as Sephiroth prepares himself and gets into the neighboring tank. Sometimes Sephiroth, who is much further down the enhancement timeline, doesn't need to soak at the same time as Cloud. In that case the man sits just beside the control panel outside of Cloud's tank and watches him float in a parody of serenity.
Cloud's memories are not under his control when he's in the tank. They rise to the surface as randomly as bubbles in a champagne glass. Sephiroth watches this, too, peering into the theater of his soul through the blanket of inky blackness. He watches, and he...comforts, perhaps. The closest word, but inadequate.
My poor Tempest, he croons, rippling in waves of warmth as Cloud remembers his rejection from SOLDIER unbidden. They did not value you as I do. They did not see your worth. Worry not. We will show them, and they will know you as my peer. They will worship us as gods, and bow to our strength. Visions of fire blood overlay the memories. They can kill every last one of the Shinra dogs that rejected and devalued Cloud. It won't even be hard.
So easy. So, so easy, to make the world fall to its knees at his feet…
No. No, never. The world is not Cloud's to own. It is not Sephiroth's to own, either. He won't do it.
No? Why not? My Perfect Storm, this tank still terrifies you so. At my side, you will never be in danger of the whims of men like Hojo. Have I not already killed him for you? Do you not like my gift?
Cloud shudders in terror, Sephiroth's voice fading for a moment as old memories of pain and desperation rise to the surface. Hojo, and surgical tables, and injections and needles and scalpels and —
And Zack.
Zack.
Zack.
Zack help me please I can't —
The darkness embraces him, and he wishes it was smothering so he could hate it, but it's not. It's soft, and gentle, and it feels so terribly good on the raw wounds of his soul. The fear is tenderly brushed aside, the terrible memories held at bay. He is a little child again, sheltering in the naive certainty of his mother's infinite protection.
Safe. He's so safe he wants to weep with relief, and curl up, and never leave.
Embrace me. Stay by my side. Be mine, and no tragedy will befall you again. You will never know the taste of fear. You will never fail, and never be failed. A vision of Zack, bleeding out and dying in the rain, fills his mind.
In this, there is a pang of honesty. Sephiroth does not know what he himself feels about Zack, his onetime loyal friend, but he has no overwhelming desire to see him dead. Zack is a useful hostage, but of course Sephiroth has no doubts that Cloud will be his, wholly and completely, once the appointed hour arrives. Zack will live. Of course he will live.
You will be strong enough to do whatever you please.
Zack rasps, "You will...be the proof...that I existed."
He will never be forgotten.
The anguish that consumes Cloud is overpowering, equal to the temptation he's resisting. Sephiroth's words are torment precisely because they're true, or at least the core of them is true. If Cloud gave in, he wouldn't be afraid anymore. He could protect Zack —Sephiroth would even allow it. He would be unspeakably strong. No further tragedy would ever befall him, because by succumbing he would already have embraced the greatest tragedy of all.
Zack's legacy would become one of fire, death, and destruction.
Cloud loses no matter what he does. He suffers, no matter what choice he makes —either directly, or by the knowledge of what his corruption would do to the world he loves. There is no end, no good option , only the faintest glimmer of hope in the distance. He might one day be strong enough to win the battle, if he bears up under the weight of temptation just one more day.
Just one more day. One step after another.
Keep going, he hears, and it's not Sephiroth. Even within the theater of his mind, it's so faint he's not sure he's truly perceiving it.
But it doesn't matter which. In the tank, barely connected to the waking world, he manages to shake his head as his eyelids flutter and twitch.
Amusement drapes across him like sticky strands of spun sugar. Silly. Stubborn. The tank starts to drain. The paralytic is wearing off. Cloud feels so exhausted he could sleep for a week, and once the mako fever that always plagues him breaks, he likely will.
My Cloud, I find your struggles so terribly endearing. The door hisses open and hands lift him from the metal grating. The amusement thickens and warps into stinging, possessive tendrils of darkness, wrapping around the core of his being. So precious. So admirable. Keep going. Strengthen your will. I will be here to help you do so every step of the way.
He shudders and whimpers like a child. Sephiroth shushes him, and lays him so tenderly into the warm bath that will wash the mako from his skin. Hands dip into the water and cradle his face, thumbs washing away the stray drops of mako that threaten to burn his eyes and lips.
Don't be afraid, my Cloud, his worst enemy says fondly. Your hunger for strength could never displease me. It will only make the day of your surrender all the sweeter.