"More than a minute in that cloud and you'll be bleeding out of your eyes, ears and mouth, praying for the pain to stop."
- — Captain Ascote, pointing out a Venomthrope to his men at the Battle of Twelve Bridges
The Tyranid Venomthrope serves as a living chemical weapon dispenser for the swarms of the Tyranid hive fleets. The Venomthrope's whip-like tentacles drip with a multitude of alien poisons. Indeed, so potent are these toxins that it is believed that a Venomthrope's very touch means certain death.
Under a Venomthrope's heavy carapace is a network of bulging, gas-filled bladders that emit yellowish spore clouds. These clouds are lethal to non-Tyranid organisms and dense enough to obscure nearby Tyranid organisms. These numerous gas-filled bladders (also the source of the toxic cloud that fills the air around it) allows the Venomthrope to move along by using its tentacle-like hooked tendrils to drag its lighter-than-air body across any surface.
These same tendrils are also used to snare enemies and drag them into its poison cloud, where it holds them fast until their feeble struggles cease and it can devour them at its leisure. Not as powerful a combatant as a Tyranid Warrior or Ravener, the Venomthrope relies mostly on its poisons to do its work, expecting only mild resistance from its prey once it gets close enough to envelope them with its cloud or touch them with its deadly tendrils.
Venomthropes are not just toxic to non-Tyranid life forms, but also to any environment they enter. Over time their miasma of toxic spores infects the ground and air of a world unlucky enough to feel its venomous touch, rendering it hostile to even its own indigenous species. The Hive Mind uses this aspect of the Venomthrope by gathering them together in large numbers and unleashing them on a region to gradually poison it, making it harder for non-Tyranid life forms to continue to resist.
Venomthropes can be especially deadly in enclosed areas or underground, where their toxins can fill ventilation systems and foul air scrubbers, making the corridors and chambers of a fortress or spacecraft deadly to its garrison or crew. In efforts to combat these tactics, the Adeptus Mechanicus has tried repeatedly to study the Venonthrope and develop either more sure methods of protecting troops against its venoms or to discover an antidote to them.
So far no successes have had any lasting effects on the Imperium’s efforts against these creatures, and as soon as a new method or antidote is found to counter the Venomthrope, it seems the creature quickly adapts and overcomes it.
A Venomthrope's feeding instincts means that it can operate quite effectively even while beyond the range of the Hive Mind, drifting into formations of enemy troops and spreading its deadly toxins or lashing out at exposed warriors.
In battle, Venomthropes are therefore often found on the flanks of the swarm, or exploiting gaps to get at rear-echelon troops where their toxic presence can cause as much havoc as possible and they can force troops to abandon defensive positions before they become caught up in its toxic vapors.
As a general rule, the only sure way to deal with such an attack is a quick response from heavy weapons and a sustained blast of fire to tear the creature apart before it can get too close. However, even after its death the Venomthrope still leaves a toxic corpse which can be almost as deadly as this Tyranid biomorph was in life.
- Deathwatch - Mark of the Xenos (RPG), pg. 51