I'll write something a bit more custom as well, but for now, some of the instances in canon of Sebastian just being the nightmare itself, since it happens consistently enough that I'm fairly sure he's able to influence and infiltrate peoples' dreams, but not concretely said enough that I would call it a powerset in an app, ya feel

Chapter 95 has spoilers if you're totally unfamiliar with the manga, but they're all about events that happen quite early on. Otherwise spoiler-free!

• Chapter 95, "Why is it that you made such a great sacrifice with even that soul of yours at stake to make a contract with me?"
  
• Chapter 101.5 (selection), "Professor, you should have no time to be writing this tale."
  
• Chapter 189, "Should you not train some more before you serve your dishes to your family?"