Brainy's mentioning that Lex should be "haunted by a Kryptonian heart beating ceaselessly beneath your floorboards" is a reference to Edgar Allan Poe's famous short story "The Tell-Tale Heart," in which a murderer goes slowly insane as he hears the beating heart of his victim emanating from under his floorboards (where he had hidden the dismembered body).
New cast member Peta Sergeant (Nyxlygsptlnz) previously appeared in Once Upon a Time in Wonderland (2013) with Peter Gadiot, who played the first incarnation of Mr. Mxyzptlk in Mr. & Mrs. Mxyzptlk (2017).