Riddle Me This: When Does Batman's Enigmatic Cipher Hunt Become an ARG?

In August 2020, Warner Bros shared the first trailer for Robert Pattinson's debut as the caped crusader at DC FanDome. Attentive fans noticed that during that trailer, the Riddler carefully wrapped up a greeting card for Batman, posing an initial challenge: "What does a liar say when he's dead?" The card went on to say

Riddle Me This: When Does Batman's Enigmatic Cipher Hunt Become an ARG?

Riddle Me This: When Does Batman's Enigmatic Cipher Hunt Become an ARG?

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The Project Gutenberg eBook of In His Arms], by Mary E. & Thomas W. Hanshew.

Riddle Me This: When Does Batman's Enigmatic Cipher Hunt Become an ARG?

Every Riddle In The Batman Explained

Riddle Me This: When Does Batman's Enigmatic Cipher Hunt Become an ARG?