******WARNING! THESE EASTER EGG INSTRUCTIONS CONTAIN MAIN STORY SPOILERS!*****
After fighting Ra's al Ghul you'll need to head back though the subway system. When you reach the room with the train cars and the armed guards, go to the left (on your map you will appear to be heading towards Joker's lair). Travel through the sewers towards the Industrial district and you will eventually hit a dead end. There is a button on the other side of some bars, and there is a gap in the bars big enough for a remote batarang to fly through. Hit the button with the batarang and enjoy a cutscene with Killer Croc.
This easter egg is time sensitive and can only be done while you are still poisoned.
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