To view this easter egg you must kill a Morgul Bat.
Once you’ve hunted and killed a Morgul Bat that can be found near the caves of Mordor, you’ll unlock some info on them in the Nature section of the Appendices.
At the end of the second to last paragraph of the entry it states that the Morgul Bats have rotting flesh on their teeth which poisons their victims and that, “The poison slowly eats at their minds, sending them nightmares of a demonic man-bat who preys on fear.”
The last line is a reference to Batman, or more specifically Dr. Langstrom from the, “The Batman” cartoon series. In the episode “The Man Who Would Be Bat” Dr. Langstrom creates a serum which turns him into a horrific Man-Bat in an attempt to make people fear him.
For those who don’t know, Shadow of Mordor was published by Warner Bros., the owners of Batman and DC Comics.