A Tyrannofex is a large type of Tyranid, somewhat similar to a Tervigon. It seems it was intended to be a big-ass creature which pounds its foe from afar (as in 48" max, which of course we know is really long range) , using a variety (i.e. 3 types) of bio-weaponry.
According to the fluff, it smells pretty bad when scared, attracting Hormagaunts and other Tyranid organisms, which then proceed to dice and shred the enemy, with their fantastically anti-terminator weapons, who will invariably be smashing this brick of an animal in Close-Combat, as it has a 2+ Armour Save and a Strength, Toughness and Wounds value of 6, and costing 175 points. Of course, being a Dakka-fex of sorts, it has only 3 Attacks and Claws and Teeth to defend itself with, as well as the aforementioned non-existent pheromone trail.
Its weapons turn it into a glorified Hellhound, a wannabe Leman Russ Punisher, and a "half a Predator Annihilator" for the Acid Spray, Fleshborer Hive, and Rupture Cannon respectively. Like the Tervigon, the Tyrannofex can shoot out spines and/or stingers from its Cluster Spines and Stinger Salvo as a secondary weapon. A running theme with these weapons will be that none of them can penetrate marine armor and none of them are going to be dealing very much damage for what you pay. The Rupture cannon's S10 and 48 inch range looks impressive until you realize it's only two AP4 shots at BS3 with no special rules to bolster it. The Acid Spray is basically the Inferno Cannon mounted on a slower, less spammable, but tougher platform (complete with additional template spewer!) that is marginally less screwed in assault, and the Fleshborer Hive is basically an infinitely worse Punisher Gatling Cannon.
On the other hand, the Tyrannofex is the last unit in the Tyranid codex that can get a 2+ without having to roll on the Ymgarl factor table, though it remains to be seen what Forgeworld will be doing with IA4's update (it's all but confirmed the Stonecrusher will go from having a 2+ save to applying a -1 penalty on all to wound rolls against it but there's a lot of other options they can add and changes to be made with the existing FW units and new units to introduce) so perhaps it will be a little less forever alone. Of course, GW may be hiding a supplement up it's sleeve that lets us put Armoured Shells on stuff again or even buff the Tyrannofex.
The Tyrannofex is also the center-piece of a dataslate formation that lets you take one of them and three gun totting carnifexes (and a mandatory warrior brood) without taking up a FoC slot while giving them split-fire. Not super-amazing but not terrible either. It's likely it will be ignored in favor of spamming living artillery nodes and skyblight swarms some more.
There's a rumor floating around that when the Tyrannofex was first drawn up, the modeling team were completely taken aback and had no idea how to actually make it. Eventually someone used a Tervigon as a base and said "close enough". One possible reason as to why they did not go with the underslung bio-cannon as shown in the 5th edition codex is because it looks like a dick and having it on the same height as the Tervigon's spawning sack would be akin to begging for shitty sex jokes to be made, so the team wisely decided to go with another location for the creature's gun. Needless to say, though, the Tyrannofex went almost four years without a model.