In Hawk Moth's lair...
"Having what's yours being taken away from you really brings out a scream of sadness," his window opens, ready to create another supervillain. "Now it's time to let out your scream."
A single butterfly lands on the palm of his hand, and he covers it with his other hand to fill it with dark energy. This process turns a butterfly into an akuma.
"Fly, my evil one, and give her this special gift," he said as he let the creature fly away outside his lair, ready to transform its chosen victim.
The akuma flies inside Boo's pigtails, causing a moth-shaped emblem to appear on her face.
"Poor little kid," Hawk Moth spoke through her with sympathy on his voice. "Did that mean, green one-eyed monster took what is yours?"
She nodded with silence.
"Don't worry, you'll get it back. Screamstress, I am Hawk Moth. I have given you the power to turn your screams into weapons. You may be a kid, but they can't silence you! However, in return for this power I've given you, you must retrieve the Miraculous from Ladybug and Cat Noir. So, ready to take back what is truly yours?"
Boo smiled with silence, accepting the deal. As a result, black and purple smoke appeared from her pigtails to cover her entire body. Once it disappears, her appearance changed. She is now a pink-skinned grown up monster with a tail.
"This can't be good, right?" Mike asked Sulley.
Screamstress launches a supersonic scream towards the former, throwing him off the wall and jumps to get his teddy bear which she believes it's truly hers. Next, she tail slams Sulley before leaving the house to wreak havoc.