From Lexicanum

Origins

The Warp is home to more nightmares then just daemons. Within the Warp exists countless writhing entities, beast of the warp, which have been born there or forged by using massive amounts of power. The Chaos Entity Tzeentch uses these beast and changes them into forms by which the material universe might recognize and fear them. Thus he creates great hungry Leviathans as pets. Creatures born of the hellish depths of imagination of mankind. Leviathans are huge warp creatures more massive then any battleship and wreathed in flame that travel the warp, just as space faring vessels travel the material universe.

Appearances

When at a point of great power from local warp storms, sacrifices, and worshipers, Tzeentch may call forth enough power to send a Leviathan traveling from beyond the Immaterium, into the material universe. Riding upon the tides of Chaos which surround the warfleets of Chaos and buoyed along by the surging waves of magic and eddies of unreality which Tzeentch followers bring in their wake.

Leviathans are followed by swarms of Screamers and other space faring daemons. The Leviathan is a giant Daemon. Such unreality not only disrupts and warps space around it, but also the Warp and the Webway as well.

Leviathans fall upon the ships like great predators, rending metal, chitin, flesh, and souls apart with equal ease. After time however Leviathans must return to the Warp that spawned them, leaving behind damage on an enormous scale.

During Abaddon's 13th Black Crusade Ahriman successfully summoned a Leviathan into the materium.

Sources

• The Powers of Chaos: By Matt Keefe & Pete Haines pg 4-11 (http://www.specialist-games.com/battlefleetgothic/rulebook.asp)
• White Dwarf 291: Eye of Terror Event Cards pg 87