That DLC is out now on Steam for £7.49/9,99€/$9.99, adding a load of new bits into the general Stellaris experience.
Today’s a big day for Stellaris [official site] as Paradox’s spacefaring grand strategy game receives both the long-awaited patch codenamed ‘Heinlein’ and its meatiest add-on yet, the Leviathans Story Pack.
And yes, 1.3 will break saves from 1.2 but you can revert back to finish games off if you want.
If you’re stuck into a game from the last patch, do be aware that update 1.3 will not work with your save file.