Canon 80D: dark corner in the image with Sigma 85 mm F1.4 EX DG HSM

This is a known problem with some third party lenses. All you need to do is go to the lens aberration correction menu and turn off the peripheral lighting correction.

This happens because Sigma copies the lens code of another Canon lens. The body of the camera sees this lens code, applies the correction it thinks it should be and gets strange patterns.

How to fix strange visual behavior with your third-party lens