Here's the revised version of your story. I've polished the flow, grammar, and readability while retaining your original voice and style:
Highgarden, Slums
Stormy nights have always been heralds of change. In many tales, children of providence are born amidst the thunder; storms seem to be bringers of transformation. This stormy evening brought together two individuals from opposite ends of the social spectrum—one, a noble of the most exalted bloodline, and the other, a child from the dirtiest depths of the lowborn.
Rain lashed against the lavish carriage adorned with rose motifs, drawn by two sturdy brown horses trudging through the muddy streets of the city below Highgarden's grand castle. The unexpected storm had delayed the carriage, its occupant being none other than Lady Olenna Tyrell, the formidable matriarch of House Tyrell. Having fulfilled her duties by securing her late husband's legacy through her son, Olenna now lived as the shadowy puppeteer, pulling the strings of power through her offspring. She glanced out of the carriage, the heavy rain obscuring the grimy streets she had always chosen to ignore.
She never liked the slums—they were a constant reminder that even in a land of abundance, the poor still starved. But for reasons she couldn't quite place, this evening, she found herself curious. Drawing back the curtain, she peered into the storm. Her sharp eyes landed on a frail figure curled in a corner, rain battering against him mercilessly. It was a boy, his curly black hair plastered to his small head, wearing nothing but a sack and barefooted in the mud. Their eyes locked. His gaze held no plea, no desperation—just a hollow resignation that unsettled her.
"Gods be damned," she muttered, closing the curtain, but the image lingered. A thought of her newborn granddaughter swept through her mind, and with a sharp command, she ordered, "Arryk, stop the carriage!"
The horses halted abruptly. Olenna muttered curses under her breath as she stepped into the muddy street, her pristine white gown instantly soiled. Her guards, Ser Arryk and Ser Erryk, rushed to shield her from the rain, but she brushed them aside.
"Damn this rain, damn these roads, and damn these clothes," she grumbled as she approached the boy. Standing over him, she asked, "Where are your parents, boy?"
The guards hovered ineffectually, their hands raised to shield her. The boy glanced at them, then pointed at Ser Arryk's shield. Olenna followed his gaze and snapped, "Use your shield, idiot!"
The knight obeyed, holding it above her. Olenna crouched beside the boy, her sharp gaze assessing him. "No parents, I take it? Damned men and their wars," she muttered. With surprising strength for her age, she lifted the boy. He was so light, it was almost alarming. Ignoring the protests of her guards, she carried him back to the carriage and deposited him on the seat across from her.
"Speak, boy. What's your name?" she demanded.
The boy hesitated before answering in a small, quiet voice, "Espada... Espada Flowers."
"Espada? Quite the name," Olenna mused.
"You saw me," the boy said, his voice barely above a whisper.
"Yes, and the rain is wet. Of course, I saw you," Olenna replied dryly. She reached for a bowl of fruit, plucking a pear and handing it to him. "Here, eat."
Espada took the pear, his dark eyes fixed on her weathered face. In that moment, the boy thought he had never seen anything so beautiful. Her face, lined with age, and her wrinkled hands seemed almost ethereal to him. He had never been welcome in the septs of Highgarden, but now he thought he understood what the septons and septas preached about divine guidance. He had found his guiding light.
One Year Later, 284 AC
The training yard echoed with the clash of wooden swords. A small figure darted around a much older boy of twelve name days. Espada Flowers, now seven, was a sight to behold. Lady Olenna Tyrell sat under a canopy she had erected just to observe the boy she had plucked from the streets a year ago. He had blossomed into a happy, intelligent child—but above all else, a born warrior.
Espada moved with remarkable precision, ducking under the older boy's swings. With a sharp kick to the back of his opponent's knee, the older boy fell, and Espada followed with a swift strike of the wooden hilt. Rising smoothly, he turned to Olenna, his grin wide.
"Did you see that, Granny? The long-necked chicken fell easily again!" he called out, drawing glares from the surrounding nobility.
Olenna chuckled, masking her pride. Espada had adopted her sharp wit and way of speech, much to the annoyance of others. "Yes, yes, boy. Now come and wipe yourself off. How many victories is it now?"
"Forty or so. Some of them aren't worth remembering," Espada replied, tossing his training sword at the feet of Ser Vortimer Crane, the master-at-arms, whose son lay defeated on the ground.
"Can I use a real sword now?" Espada asked with a cheeky grin.
"It's not my fault if you cut your pretty face, boy," Olenna retorted, her disguised grin betraying her amusement.
Ten Years Later, 294 AC, Highgarden Tourney Grounds
"By the gods, who is that? He's winning everything!"
"How is he still a squire?" murmurs rippled through the crowd as yet another opponent fell before Espada Flowers. Clad in Tyrell colors—green and gold chainmail, silver gauntlets and pauldrons, and a white overcoat bearing a giant rose—Espada was the embodiment of grace and arrogance. At sixteen, he was the most feared fighter in the Reach, though still technically a squire.
In the nobles' box, Olenna Tyrell watched with a mixture of pride and amusement. Espada's antics had always entertained her, even as they ruffled the feathers of the nobility. As he defeated his latest opponent, Espada theatrically removed his helm and bowed, not to Lord Mace Tyrell, but to Olenna.
Lady Margaery Tyrell rose and declared, "Squire Espada Flowers, my grandmother and my father have seen your skill in battle. They wish to see you joust, but as you are not yet a knight, I hereby call you to wield your blade for justice and honor. Will you, Espada Flowers, take this honor?"
Espada knelt before her, though his eyes were fixed on Olenna. The boy, now a man, was still her sharpest thorn. "I dub thee the Knight of Roses, Ser Espada Flowers," Margaery proclaimed.
Espada rose, his smirk unshakable. With a flourish, he tossed a bouquet of yellow roses into the crowd before plucking a red rose and kneeling at Olenna's feet, presenting it to her. Olenna, for all her sternness, couldn't suppress a smile.
The Reach had gained a new champion, and Olenna Tyrell had gained a son—not by blood, but by choice.