Everyone can say that almost everyone in MHA has nightmares that can vary in intensity with fear. I bet Eri has really bad night terrors being a child and her life with Overhaul prior to Shigaraki taking her. I think in the LoV probably Shigaraki has the worst nightmares out of them or Dabi. So set the scene-
It's midnight or 1 AM and everyone is asleep. Eri wakes up with a freight shaky breath, cold sweat, horn tingling and shaking. She runs to find Shigaraki.
"You can always come for me if you need anything. Doesn't matter how big or small, where I am, or what time it is. Just find me and I'll make it better," he always tells her.
She shakes him awake trying not to cry or be loud to wake up the others.
Groggy Shigaraki looks at Eri waiting for his eyes to adjust but once they focus he immediately sits up. "Eri, what's wrong?" he asks picking her up, careful to not activate his quirk.
"I don't know what happened. It felt so weird, my body was heavy and couldn't move and he was there. Cutting and cutting and cutting and and-" she cries shaking.
Shigaraki shushes her gently going to wake up the others. If he has to be awake so do they. They also would want to know if Eri got scared. He starts by waking up Kurogiri and handing Eri to her while he gathers the others.
They all gather in the main room after gathering scrap blankets and pillows. Dabi starts a small controlled fire leans against the wall. "So what happened kid? Simple nightmare? those suck," he hums.
Eri nods hugging her arms tight to her body. "It was a nightmare but it felt so weird. I couldn't do anything and it felt so real. like I was there again. Why did I deserve that? what did I do?" she asks crying.
"Oh, honey," Toga starts " you didn't do anything to deserve that. He was a mean mean man and you deserve the world."
She nods her head gently and slowly relaxes as the League tells fun nursery rhymes and short stories. Eventually, Eri falls back asleep curled in Shigaraki's lap who is gently rubbing her back or petting her hair.