dark shadow in the corner of my monitor

if it's a laptop, think back to whether the lid got closed on something. a charger cable or earbuds on the keyboard leaves exactly that kind of dark patch in one corner.

Take a screenshot and look at it on your phone. If the shadow isn't in the screenshot, it's the panel, not the PC. On a black screen, light glowing in from the edges is backlight bleed. A dark patch that's there on a white screen is more likely pressure damage or the backlight going in that corner. Either one is warranty territory if the monitor is still covered.