Venomthrope
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What the Zoanthrope is to the concept of "psyker", the Venomthrope is to the concept of "poison". Floating above the ground on organic baloons full of toxins and porting enough tentacles to make Cthulhu blush, the Venomthrope is effectively a short-range terraformer for the Tyranid invasion force. The could of spores and bacteria it emits are not only a comfortable little bubble for Tyranid units, they're a huge pain in the ass for any of those pesky planetary defenders that might have survived this long.
On the Tabletop
Shit man, the smoker got jacked! Seriously though, the Venomthrope freaking awesome and is one of the few Tyranids to come through sixth ed unscathed. (Hell, that crazy bastard got stronger!)
It's a crazy looking tentacled bastard that has 2+ poisoned attacks and gives Shrouded to anyone in a 6" inch radius. Not any model, and Unit. It's fucking crazy, just stick that guy in the middle of a meleefex brood and you have what is essentially 3+4++ carnifexes. Except against flamers, but when are Carnifexes ever worried about those? Artillery might be a problem, though.