It's 13|13.
The real kicker behind Leviathan is that it's not only finicky about getting enough creatures to set up (gotta fill up that first row), but it's also a creature that can die to permanent control (kill the Inundation). It's a huge gamble both in terms of comboing it into existence, but also in terms of keeping it alive.
That said, the cost-benefit is HUGE, and you get to Flood your opponent along the way.
I'm about as fond of this idea for
as I was for Alpha Wolf and
. I think it's thematically appropriate, cleverly balanced, and edgy without being OP. But enough modesty on my part.