The Troll Hall Chronicles
Pyramid of Shadows
The Pyramid of Shadows is an extraplanar prison which was originally constructed by demons for reasons unknown, although its chief inmate appears to be a tiefling wizard named Karavakios. Our heroes came to it at the end of their Season 1 adventures, being sucked through a hellish portal.
How it came to be, what it might fully contain, and how they might escape, are all mysteries our heroes have not yet solved.