Hierophant
The Hierophant is a Forge World-only fuckhuge Tyranid biotitan which is a fucking engine of destruction. It has a large arsenal of bio-weapons and is the Tyranid response to Gargants and Titans. The largest playable Tyranid unit in the game, it stands at 10 inches without a stand. However, in the fluff it is 15 meters tall. To put this to scale, the average height of a Imperial Guardsmen stands at around 1.5 to 1.8 meters tall while a average Space Marine is 2.5 to 2.7 meters, so on the table you can expect the size difference between a Hierophant and most things to be approximately fluff accurate - in other words it's pretty fucking huge.
They also weigh at around 51 tonnes, which in real world terms is not actually that massive. Compared to many sauropod dinosaurs (not to mention things like tanks), this is rather light. Furthermore it would be a rat compared to the Blue Whale. Figures that GW would end up failing at scale again.
With that in mind, the Heirophant is probably not all that dense. While the dinosaurs prove that land creatures can safely get that large, dinosaurs have legs under their bodies like pillars, while the Heirophant's front legs legs go up to the "knee" then down again. From a structural perspective the body is hanging off the limbs. Additionally, for the Hive mind to create a creature that can only invade Earth-sized or smaller worlds is likely be a waste of time (unless it turns out that larger worlds in 40k don't allow the kinds of fortification that need a Hierophant to attack). Instead of being a huge brick like a Carnifex, the Heirophant is probably light for its size, armored and structured in a self-supporting way to enable so it can either take cannon shells without breaking bones or splitting armor, or it can suffer broken bones and split armor but can those wounds can't kill it before it can perform its mission. Purely speculation, but one could assume it has legs made entirely the hard chitin Tryanids use for armor, with muscle and bone only extending far enough to give mobility and reinforcing structure; Terran life forms do this as well - it's why chickens have big drum sticks but much less meat on the rest of their legs. The inside of the beast would most likely be entirely metabolic structures and weapon organs (standard Tyranid fare), but with a dense and powerful digestive system in the lower front area to take in energy - remember that petroleum, what we use for fuel, is just plants that have been changed by the earth, and it has a very high energy density. As such, it's not to hard to imagine the Hierophant being powered by some kind of organic gasoline, though it would need a lot of fuel to create enough energy-dense petrol; thus, either it needs to conestly "nurse" off a hive ship or the hive mind just has hormagaunts jump in its mouth to give it enough energy to fight. The supportive structure is likely a porous, hard (and regenerative) supporting material with a dense bone-core and plated chitin armor, segmented like an insect. The odd shape of the body probably protects vitals from infantry, smaller beasts, and flame weapons; it can walk over barricades and could even use it's circulatory system to move heat generated from movement or energy weapons to it's top, ensuring weapons organs function at peak efficiency. Alternatively, it might be at least partly supported by the hive mind, like a Zoanthrope.
Of course, unless its weight is mitigated by psychic hijinks or it being filled with helium (or better yet, a considerable vacuum), there's no explanation for how a 51-ton creature on FOUR POINTED LEGS doesn't sink balls-deep into the ground the moment it makes planetfall.
It's also worth noting that all those giant dinosaurs were very slow beyond being able to take a large stride, and most large Earth animals on land have to actually be extremely cautious with their movements because it's surprisingly easy for them to break bones from relatively small falls due to their sheer mass. Even at elephant size, a slip down a few meters can cause some pretty serious injuries. While for the giant sauropods high speed movement would be next to impossible and so much as tipping over would likely be fatal. Meanwhile, the Heirophant is noted to be extremely fast (easily capable of outrunning people with jetpacks) and shrugs off tank shells like annoying gnats and can easily get into high speed rough and tumble melee fights with Titans and Stompas so even with Forgeworld's rather low weigh estimations, the Hierophant is already a very much impossible beast thanks to its sheer agility (being far faster to react and move and substantially more agile than most smaller animals) and its incredible resilience. A bruthakayosaurus may be bigger but it's more fragile, slower, and would have had to spend a great deal of its day doing nothing but eating to sustain itself while the Hierophant can run around at high speeds over the course of a Hive fleet's assault on a targeted planet.
Crunch-wise, this is the most powerful Tyranid model in the game. It's extremely durable, throws out more attacks per turn than any single model - besides a CSM model that gets lucky a demon weapon, a Dreadnaught with blood talons since it can more attacks, and An'ggrath if rolls lots of 6s - and packs two S10 biocannons, which are sadly only AP3. It's only BS3, but with that many shots it's not going to matter. As the number of attacks it has, plus the fact that it's a gargantuan creature, would indicate, the main area it excels in is close combat: Apocalypse exclusive Daemon-lords are likely to die against it. It's damn hard to kill, too, having 10 wounds, toughness 9, and can regenerate and get a wound back by rolling a 6, and it gets to roll a six for every remaining wound it has.
Phil Kelly has one, and he called it Nancy. 'nuff said.
8th Edition
Holy fucking christ.
Listen. The Hierophant has always been a great choice. Although it was extremely expensive, it always pulled it's weight. The only problem was that the Tyranids never had a true badass TITAN. At the end of the day, the Hierophant was just a gigantic 'Nid.
Then 8th Edition happened and turned that shit RIGHT AROUND.
The Hierophant Bio-Titan is just that. A bio-TITAN.
3+-5+ WS, 3+ BS, S10, T8, 6-3 Attacks, an armor save of 2+ and an invulnerable save of 5++, all while moving from 12"-3". You may of noticed I skipped the wounds. We'll get to that. Bio-Plasma Torret is pretty much a Flamer with 2D6 hits. Did I mention the Pistol rules? It's Bio-Cannons are now... Dire Bio-Cannons? Whatever, the stats are pretty much the same, but the AP is now -2 and the Damage is 2D6. And yes, you still get two of them. They are Macro 6 weapons, basically meaning Titanic weapons. So yes, 12 S10 AP-2 2D6 shots on a single target is possible. And if that just was not enough Dakka for you, for the value price of D3 Mortal Wounds, you can double the strength of your bio-cannons. See also: Rape.
The Hierophant also has some great Melee options as well, with Lashwhip Pods and Monstrous Scything Talons.
Although the Lashwhip Pods only do 2 Damage, you get 3 hit rolls for every Attack. You wanna use this against big crowds. The Monstrous Scything Talons have Sx2 (That's 20 fucking Strength), AP-5 (No Armor saves whatsoever), and, yet again, a Damage of 2D6. 6 attacks of S20, AP-5, and 2D6 Damage is most likely a kill to damn near anything that looks at you funny.
You also can get Incendiary Ichor (It's just Acid Blood) or Swarm Incubation Chamber (Can carry 20 Gaunts or Genestealers/6 Hive Guards, Warriors, or Tyrant Guards, AND a Prime or Broodlord. An example would be 20 Genestealers and a Broodlord, or 6 Warriors and a Prime.). Both are free.
All that for just the low, low price of- Shit. It's now 1800 pts. AND a Power of 90.
Oh yeah, the wounds.
50. The Hierophant is now 5 times the Lord of War it once was.
So, worth it? Probably. Just sad we won't be seeing Hierophants in non-apocalypse games, even though they were usually reserved for apocalypse games anyway.