What the Zoanthrope is to the concept of "psyker", the Venomthrope is to the concept of "poison". Floating above the ground on organic balloons full of toxins and sporting enough tentacles to make Cthulhu blush, the Venomthrope is effectively a short-range terraformer for the Tyranid invasion force. The cloud 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. Shares the same Feeder Tendrils as the Lictor as well as a set of Ripper Tentacles usually found on the squatted Mycetic Spore. It relates to the Toxicrene in roughly the same way as the Zoanthrope does to the Maleceptor, except that the Toxicrene does not suck.
On the Tabletop
The Venomthrope has 2+ poisoned attacks and gives Shrouded to all units within a 6" inch radius. This makes them extremely useful as support for your masses of gaunts so that they have at least some kind of save. This also works great for supporting your Carnifexes and Hive Tyrants alongside their followers: that cover save is a blessing when dealing with AP3 weapons. This obviously makes the Venomthrope a premier target for the enemy forces: especially Tau are laughing as they ignore your cover save and one-shot your entire brood with a powerful weapon. Template weapons are dangerous as well: a few shots with the equivalent of a Heavy Flamer will do a lot of damage on your Venomthropes. But as long as you keep them in the back and protected by units that can devour whatever unit that comes near it, discouraging suicide tactics a bit. And make sure you take out the opposing artillery ASAP.
Venomthrope still has a nice benefit: enemies that shoot at shrouded Tyranids suffer -1 to their Ballistic Skill. This is NOT ignored by anything that Ignores Cover, cover in this edition applies +1 to saving throws instead. Although flamers still auto-hit, so there's that to worry about. Now require a brood of at least 3 if you want the sporecloud to affect your monsters, and a full brood of 6 extends its cloud to a range of 9". Overall these cuties are now less useful than they were in 7th edition and are generally outshined by the Malanthrope, but they're still playable.
Losing most of their offense abilities in 10e, Venomthropes have become a strictly defensive unit. And what a defensive unit they are! Any friendly Tyranid unit within 6" has Benefit of Cover, and all non-Monster units within that range also have Stealth (which the Venomthropes have built into them). This can make any unit annoying to kill, letting you close the distance in relative safety. The Venomthropes themselves have been nerfed a bit: down to 6" of speed with all their combat abilities being in their Toxic Lashes: 5 attacks at 3+ and Anti-Infantry 2+ makes for an average of 8 wounds... but at AP0 and D1 you'll break your teeth on anything even close to a MEQ. Out of all Hyper Adaptations the Sustained Hits 1 is their best bet, but against a softer target the Precision one has its uses. In turn, T5, W3 and a 4+ save means that while they can prevent incoming damage they can't handle being hurt very well. Remember that in a tough situation they can Fly, so you can abandon your post if they are threatened.
And how much do you pay for all of this? 70 points per three, a 33% cut from 9e. Awesome.