What if Yuna had agreed to Yunalesca's proposal? (Note: this one-shot is being expanded/revised into a multi-chapter epic, "Love Her and Despair;" it's got spoilers to that fic!)
On the tallest of these, Summoner Isaaru and his Guardians were struggling to fight despite the maelstrom and the wiles of their foe. Sin's heart beat with deadly purpose, toying with them as a coyote played with its food. The railings and stairwells of the huge observation deck were burning, but between them a rugged landscape of broken ice had covered over the open space. Great spears of frost leapt up and disappeared underfoot as they dodged and reeled, and they could not stay together. Lightning howled and spat at them. Burning embers tossed from the inferno raging around the city rained down on them or the icy surface, burning limbs or creating slick puddles.
The source of this mad frenzy of elements was an exquisite statue of a woman set at the exact center of the tower like a soaring pinnacle. The figure was composed of a curious webbing of black iron plates and veins of translucent white ice. It held more than a passing resemblance to Shiva, if Shiva were forged of meteorite's ore as well as ice. The image never stirred, hands clenched at sides, chin frozen at an arrogant tilt, fine-chiselled features so exquisite that they were illuminated again and again in temple writings both for and against what Sin represented, and served as illicit imagination's fodder for novice monks in the loneliness of their cells.
A faction had sprung up in Luca in the last two years that had broken away from Yevon and declared that Sin should be worshipped for the goddess it was. Who could deny that it had changed since last Calm?
No one, but Isaaru had made a promise at the dedication of Yuna's statue in Bevelle that he would be there to avenge her and her companions should Sin ever dare return. He had thirteen extra years to train. Now he needed all his strength to heal the trio of warriors fighting at his side.
The three brothers had been separated by the maddening ice-crystals that kept shooting up from below, and Pacce was lying in a red pool on the frozen ground which had betrayed him a few moments ago. Isaaru was trying to reach him. Maroda had been chased nearly to the edge of the platform by the shooting pillars and flashing bolts, but was unharmed.
The silver-haired older Guardian, shoulder laid open to the bone, was ploughing doggedly towards the statue at the center of the madness. He weaved drunkenly around the ice spears, huge sword scything through the strange whipping tendrils of black leather and toothed metal flailing out from the statue's head like maddened snakes.
Another lightning bolt sizzled downward. He swerved, weary frame aided by haste-fueled magic, and a blue nimbus danced across the rim of the old Guardian's glasses, but the brunt had missed him. Again.
"Two hundred," he said with tired triumph.
A gale-wind blew across them, and Maroda went skidding. The thunder faded. Flames still leapt and water still gnawed below at the tower's knees, but the lightning stalagtites and ice stalagmites had vanished. "What the...?" Pacce groaned as he reached for his spear.
A moment's calm. The grizzled swordsman faced the glorious figure that towered over him, unmoved by the interplay of light and shadow or the face that captured the beauty of one he had once known. In his mind's eye he saw only iron bars. Sir Auron drew back his sword and hurled it with all his remaining strength towards the seam between dark and light bands, striking it just above the breastbone. His weapon sheared through. With a scream of metal and a glassy explosion of ice shards, the shell cracked open, hurling shrapnel at them as it disintegrated. His glasses, high collar, and cuirass shielded him, but his arms and legs were bleeding as he lunged forward to catch the small figure in tattered leather who crumpled backwards, blood falling in rhythmic pulses around the sword-blade.
"Isaaru," Auron called hoarsely. "Life. Hurry."
Something huge and black and gleaming exploded out of the woman's body and went hurtling upwards, disappearing into the roiling clouds overhead.
The Summoner hurried forward. "You're sure it's not wishful thinking, old friend?" he asked with gentle sternness.
Stonefaced, the Unsent warrior yanked his sword free of the ghastly wound, and the blood leapt out from the woman's body. Her face was totally veiled by her wild mass of hair. "Yu Yevon has released her. Quickly, before it attacks."
The quiet man that reminded him so much of Braska gathered himself, and a rain of white feathers and holy light absorbed the blood into itself, sealed mortal wounds, and coaxed breath to rise and fall beneath her breast again. Her dark lashes fluttered open.
"Late again, Auron," Lulu whispered fondly.
His lips brushed against her forehead. "You gave us quite a Trial. Can you stand?"
"You brought no mage?" she queried, gripping his shoulders as Isaaru cast healing over both of them.
"And no Aeons, my lady, as per your advice," the Summoner replied. Two years ago, Sin had started methodically attacking every shrine where a statue of the Fayth was housed. Yojimbo's was the only one left, entombed out of reach by a massive rockslide. "You will be the last Sin. I vowed it to Lady Yuna's statue."
"Yuna." The bleakness was back in the woman's face, a smoldering fury that had seen Besaid wiped from the map by fire and water six months ago. "I have waited a long time to avenge her."
A twisting, writhing dark shape hurtled down out of the clouds. Auron lifted the mage to her feet and turned, retrieving his sword. "Now we avenge them all."