The Stick and the Serpent is a symbol of the Plague Doctor. It usually appears on places where the Plague Doctor has visited, though no one knows who puts it there or why. The symbol is often tattooed upon Oathbreakers, usually in a hidden place. The greater Oathbreakers, the Mad Doctors, might not actually have it tattooed, since they are better at hiding in plain sight.
The Stick and the Serpent is said to represent a twisted version of the Rod of Asclepius, which represents healing and hospitals.