Hive Fleet Leviathan

Leviathan is the third and largest hive fleet so far to have assault the 40k galaxy in Warhammer (you call tell by mentioning it as the largest so far GW is just planning to unveil a new bigger fourth one as a global summer campaign at some point). The hive fleet has struck below the galactic plane, rising upwards in a jaw-like formation swallowing whole entire sectors in the shadow of the warp. Holy Terra lays directly in their path and for a chilling reason; the Emperor, glowing as he does in the warp, is serving as a big beacon to the Tyranids which they can't miss. In fact the canon pretty much says he is responsible for bringing the Tyranids to the galaxy in the first place because he is acting as a draw to them. Way to go Emps.

Inquisitor Kryptman, the long standing authority on the Tyrannic menace, quickly acted when he suspected a new hive fleet was arriving and did several things that while are extreme also make a hideous sense of logic. He ordered the exterminatus of countless worlds to deny the Tyranids vital biomass, really hurting them the only way how. The exchange of this though was countless numbers of people have died and inhabitable worlds have been destroyed wholesale. Even the Imperium couldn't stomach this and the Inquisition has kicked Kryptman out. He was not done yet though. By capturing a brood of genestealers and sending them off into ork controlled space, he hoped the two species would destroy each other but it seems to be backfiring amazingly, as both sides seem to be growing constantly stronger through the conflict. Whoever wins may end up being a threat greater then either of both of them posed....d'oh!