Toxicrene
The Toxicrene is a monstrous Tyranid designed as a more offense-minded Venomthrope. Spotting a dozen Lash Whips all filled to the brim with a hideously effective poison (D3 damage, reroll failed to wound) it is quite effective at dealing with tough enemies. Its model is very large: the massive tentacles make the Toxicrene nearly three times as wide and one and a half times as long, encouraging the use of pinning to be able to remove the tentacles while in transport.
It was formerly marketed to be effective against Terminators and powerful Monstrous Creatures like Riptides. None of the Tau players took this seriously, a scoffing not entirely undeserved when you consider the Toxicrene is footslogging while Riptides are faster with jet packs and have easy ways to ignore cover. That said, if it DID get in contact with an enemy Monstrous Creature or tough units like Terminator squads, especially those running with Apothecaries/Sanguinary Guard it hurt, making this a DISTRACTION CARNIFEX at worst. Nowadays its use has been reversed: while it used to be effective against units with high toughness and armor but less so against vehicles, it's now effective against tough units and medium vehicles but less so against enemies with 2+ saves.
Tabletop
On the tabletop the Toxicrene is an odd but potent beast. With 12 wounds, T7 and a 3+ save it can hold its own for a while, but because of its close-ranged nature it has to get in a fight quick. All attacks with its tentacles are S User, AP-2, D3 damage and can re-roll failed to wound rolls. Paired with its high strength and average WS means that a Toxicrene will not hit all that much, but when it hits the thing stays hit. This makes a Toxicrene a non-ignorable threat to vehicles, especially when it also swings its tentacles for a ranged attack. On top of all that it also gets to launch poisonous spores for a D6 S3 AP0 D3 damage attack that ignores cover, it can deal a Mortal Wound on a 6 if it gets wounded in close combat or just by being within 1" of an enemy model (and this goes for EVERY enemy within 1") and it flips it shit if it dies and you roll a 6. If you can get it close it can deal quite a bit of damage, but beware tarpits and enemies with high armor saves because they will ruin your day. Don't make it do jobs it's not fit for: let Exocrines and Tyrannofexes fight at a distance while Carnifexes deal with the heavier targets.