Leviathan is a shadowy organization with origins unknown, capable of creating surgically and genetically altered super-humans. They've also shown an ability to brainwash people for their cause.
The head of the organization was once unknown, until it was later revealed to be Talia al Ghul. One notable agent of Leviathan is The Heretic, a mysterious figure Batman and Damian Wayne came across in Yemen, dressed in Batman inspired Arab garb. When Damian initially discovers him, The Heretic gives him a haunting message of "You will know me. But not yet."
Thanks to a tracker placed on The Heretic by Damian, Batman first discovers Leviathan by listening into a meeting where they declare war on him. His response, "Game on."