Leviathans is easily the worst story pack of the entire game. It doesn't really adds much, and the main features of the pack aren't even related to the name "Leviathans". It's just sounds like a small pack of random ideas that the developers were too lazy to implement into other dlcs. Like for example the War In Heaven situation, that is by far the best addition of the dlc doesn't add nothing about leviathans and could fit much more if added into Utopia, Nemesis or Federations.