Leviathans were immense transcendental beings which dwelt in the Void between universes.
An average universe was approximately 157,000,000,000 light-years in circumference. In comparison, a typical Leviathan was 150,000,000,000,000,000 light-years long, making them over 3 million times larger.
Little was known of the biology of these creatures until a Seventh Wave cohort cut off from the Homeworld settled temporarily in the region of the Anvil Stars. There they witnessed the budding of a proto-universe and saw that certain mechanisms (which they referred to as "gardeners") sorted through and separated the universe seeds. They discovered that these gardeners were turning some of them into baby Leviathans.
This knowledge was suppressed, but agents of the Celestis managed to steal a datacoil containing the information. (PROSE: The Book of the War)
Iris Wildthyme had to avoid Leviathans when she drove through the interstitial space between universes. (PROSE: Flasket Brinner and the Clockwork Heart)
Leviathans disturbed and shifted reality merely by existing, to say nothing of moving. (PROSE: Spinning Jenny)