This TUS stands for Traditional Unicorn Sleepover. Misty and Sam(Your OC.) are invited to their first Traditional Unicorn Sleepover by Lzzy. It's a tradition whatever you make your very first best friends. Sam and Misty asked Opaline if they can come to the sleepover.

Misty:So I get to go to the Party?!

Opaline:Yes,you two gonna go party up alright. You two needed to take a break from studying magic anyway.

Misty:Really?

Opaline:Yes. I feel like I'm putting pressure on you two so Yuppers.