Mark stood in the hallway, staring at the door of the twins’ bedroom. His heart was pounding in a way that made him feel sick, the kind of sick you get when you know something terrible is coming but can’t do anything to stop it. The house was dark, save for the faint glow of a nightlight seeping under the door. That soft light was somehow worse than total darkness—it hinted at things unseen, horrors lurking just beyond perception.

He didn’t know how it had started, but he knew when it had gotten bad. Ever since their seventh birthday, something was off about one of them.

One of his boys.

It was little things at first—things only a father might notice. A slight difference in the way one of them smiled. A small shift in how one of them spoke, like they were testing out new vocal cords. Their favorite foods swapped overnight, like someone had turned a switch. And then, the nightmares began.

Mark’s wife, Karen, dismissed it as sibling rivalry, a phase. “They're identical twins, Mark,” she would say with that forced, placating tone. “They’re bound to mix things up. You worry too much.”

But that wasn’t it. It wasn’t. Mark knew. One of them wasn’t Danny or Luke anymore.

The real terror began three nights ago. Luke had woken up screaming, his tiny face twisted in terror. When Mark rushed into the room, Danny sat on his bed, watching. Not moving. His face blank, eyes wide, reflecting the flicker of the nightlight. Mark had scooped Luke into his arms, whispering that everything was fine, everything was okay. But the whole time, Danny had just stared. Not a word. Not a tear.

Something wasn’t right.

And now here he was, standing outside their room again. He needed to know. He had to know. His fingers twitched at the brass knob, the cold metal sending a shiver through his arm.

Slowly, he turned it.

The door creaked as it opened, revealing two small figures in their beds. Danny on the left. Luke on the right. Both boys were sound asleep, faces angelic, the very picture of innocence. Mark’s breath caught in his throat.

How could he know?

He stepped into the room, his eyes scanning every inch of the boys, searching for the answer that had been tormenting him. Was it Danny? The one who had stared so coldly the other night? Or was it Luke, who had cried out for him with such terror?

A low creak came from behind him, and Mark froze. The closet door, once barely ajar, now yawned wide.

His heart skipped.

“Daddy?” a voice whispered, high and tremulous.

Mark spun toward the sound. Both boys lay perfectly still in their beds. Neither of them had spoken. His skin crawled as the hairs on his neck stood up. He didn’t want to look back at the closet, didn’t want to see what might be inside.

But he had to.

Slowly, he turned his head toward the dark gap.

"Daddy," the voice whispered again—this time from behind him. Right behind him.

Cold breath brushed the nape of his neck.

Mark whipped around, eyes wild. The boys were still asleep. Motionless. Innocent. He stumbled backward, feeling his way to the wall, his breath ragged, chest tight. He couldn’t take it anymore. One of them wasn’t his son. One of them was something else, wearing the face of his child like a mask.

He fell to his knees, fists pressed against his forehead, fighting to control the sobs that threatened to escape.

And then, ever so slowly, Danny’s eyes opened. He smiled. That strange, off-smile. Like he knew something Mark didn’t. Something awful.

The room felt colder than before.

“Daddy,” Danny whispered. “Are you scared?”

Mark’s voice failed him as his heart pounded in his chest. Behind him, the closet door creaked shut.

And Luke smiled too.