The word "Leviathan" in Leviathan Blade, comes from a sea monster called the Leviathan, mentioned in the bible, Job chapter 41.
The whole chapter goes to describing it, so I suggest reading that if interested.
There is one verse in the chapter that reminded me of the actual in-game Leviathan Blade:
"I will not fail to speak of his limbs, his strength and his graceful form." (verse 12)
This describes the Leviathan Blade pretty well, IMO. :D
Anyways, just thought that was interesting!
By the way, fail picture of the day:
http://img856.imageshack.us/img856/6442/spiral20111026190753.png
Leviathan has been a name used since the Final Fantasy days, maybe even prior to that with old PC adventure games, maybe you are just bringing it up since Spiral has the name as well. It's supposedly one of the dragons/dinosaurs that Christians said existed back then to prove that dinosaurs were living with humans(some even claim they exist in remote swamplands(like the Lochness monster or ones in African swamplands). Another one is Behemoth. Anyway, I'm not sure if its origins is in hebrew scripture or prior to that with mythological folklore in whatever country cause it has been translated through greek or latin.