I searched around for about 30 minutes but couldn't find the triangular piece with the circle drawn on it...
Also, I don't know if it's a bug or a design issue, but if you get too close to a wall, the screen will turn black.
On the right side of the room (as you're facing the mirror), there's a pot placed up on a ledge, the circle ("Ouroboros") tablet is in there. Grab a rock and throw it at this pot to break it.
Thanks for pointing out the black screen bug. I think I know what's causing this and will fix it in the next update. Thank you!