The Ouroboros is a massive, round disc a little taller than Feyre in a bronze casing. The metal frame around it is fashioned after a serpent devouring its own tail.
If someone was to look into the Ouroboros, it would show them their true self, not how they look on the outside but how they are within. It drove everyone who looked into it crazy until Feyre went to get it for the Bone Carver. Although it was a terrible experience for her at first, she overcame the image of herself and learnt to accept it.
The Ouroboros is an ancient symbol depicting a serpent or dragon eating its own tail.