When in the King's Domain, if you head behind his portal and stare horizontally at it while moving behind it, your screen will glitch, either flickering or turning black, and your GUI will have trails of itself. This was tested 3 times, once when initially found, once by Slick, and once again by me when I relogged.
Link to evidence and recreation below.
https://giphy.com/gifs/ZcQCVhYoHGJVC3lGCc