"A post written by user Roko goes viral on social media, reaching an audience of several billion people. Roko hypothesises that, in future, there will be a superpowerful AI known as Roko's Basilisk, created in order to optimise human society.
The Basilisk would be supremely powerful - it could not only subject you to extreme torture, but could generate billions of perfect simulations of you and subject them all to horrendous torture too. In a means to ensure its own creation, the Basilisk would do this to any individual who knew the Basilisk might exist, i.e. who had considered it's possibility by reading the post, and chose not to help bring it into being.
So, if you chose not to try to help bring the Basilisk into being, you and billions of perfect simulations of yourself would be tortured eternally - if the Basilisk came into existence, that is"