Hexakosioihexekontahexaphobia is a fear of the number six-hundred sixty-six. People who suffer from this fear are referred to as hexakosioihexekontahexaphobes.

This fear most likely stems from a concept (the Number of the Beast) found in the New Testament, where this number represents some seriously bad news - the antichrist, the four horsemen of the apocalypse, some kind of zombie outbreak, etc.

Additionally, 666 is a triangular number.