Here is a polished and expanded version of your scene. I fixed the grammar and tenses, enhanced the dark, stormy atmosphere, and added the extra 100 words to deepen the tension just as they open the door to the abandoned room.
In every world, there is a hero who rises to kill the Demon King. In our world, our hero did exactly that—he struck down the ultimate evil. But the story did not end there. In fact, it was only just beginning.
Every millennium, a new Demon King is reborn. Weary of the endless cycle, the legendary hero decided he would no longer fight these battles himself. Instead, he chose to raise a successor—a disciple worthy of inheriting his mantle. To aid him in this quest, the God of our world granted him a temporary, fleeting life to find and train the next generation.
RUMBLE!
In modern Japan, it was a fiercely stormy night. The wind howled through the streets like a screaming whistle.
KABOOM!
A blinding streak of lightning struck a nearby utility pole, causing the transformer to erupt into flames. Instantly, the lights cut out, plunging everything into darkness.
"Big brother, I'm so scared! Why did the lights go out?" Miran Takanashi cried, rushing forward to grab her brother Kohaku's back.
"Don't worry, the power will come back soon," Kohaku said gently, trying to comfort his sister. He clicked on a handheld flashlight, casting a dim, flickering beam that filled the hallway with long, creeping shadows. He sighed, looking toward the window. "Why is the weather so angry tonight?"
In a half-crying voice, Miran asked, "Where did the director and the other children go?"
Kohaku and Miran were twins. They had lost their parents when they were only four years old, and because none of their relatives wanted to adopt them, a local orphanage had taken them in.
"They are all down on the first floor," Kohaku answered, patting his sister's head reassuringly. Both of them were now sixteen years old, and tonight, Kohaku had a secret mission. He had to go to the restricted attic. But with his terrified sister clinging to him, it was turning out to be incredibly difficult. This was his burden to bear, not hers.
Half an hour ago, Kohaku had looked at his sister with dead serious eyes and said, "Miran, I don't think I can stay with you much longer."
"What do you mean you can't stay with me? Do you want to leave me all alone? Where are you going?" Miran had asked, her voice a painful mix of confusion and heartbreak. Kohaku couldn't bring himself to tell her the truth.
"I'm not leaving forever, but... it will be a short departure," Kohaku had lied, trying to make an excuse.
RUMBLE!
The storm outside only grew worse. Miran had always been a crybaby, and she absolutely refused to let her brother go anywhere. Kohaku desperately wanted to say a proper final goodbye, but time was running out fast.
Back in the present moment, Kohaku looked down at his crying sister. "Why did you follow me up here if you're so scared?"
Miran held onto his hand even tighter, her knuckles turning white. "I won't let you go anywhere! I know you're trying to leave me behind. I don't want to be all alone!"
Lost in his own thoughts, Kohaku whispered under his breath, "This isn't right... but I don't have a choice."
Miran caught his words and looked up, confused. "What do you mean you don't have a choice?"
Kohaku didn't answer.
A few minutes later, they stepped into the deepest, darkest corridor of the building. "We're here," Kohaku muttered. They were standing directly in front of a heavily sealed, abandoned room that the orphanage rules strictly forbade anyone from entering.
"What's inside here?" Miran asked, her voice trembling as she stared at the rotting wood of the door.
Kohaku reached out, his hand gripping the cold metal doorknob. With a heavy groan of protesting hinges, he pushed the door open.
The beam of his flashlight cut through a thick layer of swirling dust, revealing a room frozen in time. The air was suffocatingly cold, smelling of ancient parchment and ozone. In the center of the room, sitting atop a cracked stone pedestal, was a bizarre, glowing artifact that looked like a shattered fragment of a sword. It pulsed with a faint, divine light, perfectly synchronized with the thunder shaking the building outside. Miran gasped, taking a step back as the strange light reflected in Kohaku's determined eyes. He knew his normal life was officially over.