The colored markings on the face of skekGra the Heretic are based on unused Skeksis concept art created by Brian Froud for the The Dark Crystal (1982).
Although urGoh the Wanderer did not appear in the original The Dark Crystal (1982), a much younger version of him previously appeared in the graphic novel "The Dark Crystal: Creation Myths" Volume III, published 2015.
urGoh's chanting during the "opera" resembles Tuvan throat singing, particularly Khoomei.
According to Jeffrey Addiss, the idea behind skekGra during the making of The Dark Crystal: Age of Resistance was that a Skeksis would have been in the best position to reveal the true history of Thra to the show's heroes.
The title is one of Chamberlain's lines from The Dark Crystal (1982) when he attempts to become emperor.