Carnifex
A Carnifex is a monstrous Tyranid creature from the Warhammer 40,000 game. A huge and terrifyingly strong monster, the Carnifex often serves as the primary damage-dealer of a Tyranid army list, and is among the most customizable of Tyranid units, allowing for many different varieties of Carnifex to fulfill different roles on the battlefield. They can even be upgraded with regeneration - a mutation previously unique to a Carnifex special character known as Old One Eye, but later introduced to the standard Tyranid army list - making them extremely difficult to kill; It's pretty much a diet Robo-Tarrasque. 40k players commonly name specific variations of Carnifex by taking another word that represents the concept and appending "-fex" to the end - for example, the Dakkafex is a shooty Carnifex based on the Ork term "dakka".
More tricky Tyranid players may attempt to use Carnifexes as decoys to draw enemy fire when their smaller units actually present the greater threat to an enemy, as opposing players are apt to focus fire on the Carnifex to attempt to destroy it before it can do serious damage.
Carnifexes in 5th Edition
In the 5th edition, the Carnifex lost a total of eighteen biomorphs and weapon options, having been replaced by more stringent mandatory loadouts. The Carnifex has also faced a doubling in base point cost with rather paltry statistical increases - putting it arguably somewhere between 20-30 points too expensive for the overpowering majority of competitive army lists, and 10 or so points above the cost-effectiveness ratio provided by variant 'Fexes (such as DISTRACTION CARNIFEX).
How profoundly infuriating this is for Tyranid players writ-large cannot be denied, and is notable for being one for being one of the first times that players openly called foul on Games Workshop. Beyond any doubt, the Carnifex is the mainstay big fucking unit of Tyranid players and is the one unit that literally every Tyranid player has in their army list. It was the epitome of ubiquitous; you could mount lots of options on it, and all of them, to some degree or another, were viable - until this edition hit.
Some of the new biomorph weapons and options were nice, and there was options added to make some of the 'Nid swarm a little less vulnerable to being dramatically outgunned, and considerably improved Tyranid psyker units (its command units especially), but in the process, the Carnifex - the one fucking model every 'Nid player and their grandmother had at least one of - got toned down dramatically and is now rather inefficient - as well as much more vulnerable to being taken down quickly by certain units with reasonably-effective armor-penetrating weapons(of particular note: Guardsmen with Missile Launchers, Hunter-Killer Missiles, Rubric Marines, Necrons, and Stormtroopers, since the fucking thing can't take Extended Carapace anymore.
It enraged players even more that this edition's weapon changes functionally forced people to buy all-new carnifexes since the old ones they fielded, such as Sniperfexes (which were armed with Venom Cannons and Barbed Stranglers, to give the Tyranids much-needed fire support and allow them to stun enemy vehicles into submission) were no longer valid, whilst several of the new bioweapons, whilst not bad (people like that the Venom Cannons can score penetrating hits now) are specifically designed to take bites out of the Carnifex's originally-legendary punch (since the gun now replaces 2 hands). Functionally, the Carnifex is now half as good at ranged support as it was before with only token upgrades in return and the loss of about 33% of the Carnifex's durability, since it can no longer shrug off a lot of weapons it used to...
Extensive debate on /tg/ has been had since the update, arguing whether the new Carnifex is a direct result of corporate evils or the result of gross incompetance or prejudice on the part of Robin Cruddace, the 5th editon codex writer. As of currently, smart money is on the former; it's widely known that GW is bleeding money due to its fuck-ups, which indicates that this may have something to do with it, especially since Robin Cruddace isn't exactly the biggest fan of Tyranids. Which sort of makes him a reverse Matt Ward.
Again, there are a few bright spots. 3rd edition's Old One Eye is back, even if he does now cost more than a Landraider and is barely better than a standard Carnifex with no weapons courtesy of Old One Eye's close-combat only biomorph loadout (his primary advantage is that he recovers wounds of fives or mores instead of just sixes, which isn't worth it at all.) The new Hive Guard is a great unit for fucking with Communist Dipshits that like to use the old Fish of Fury tactic. Wider support for variant weapons, previously from Chapter approved, are also a plus. It's a shame that the good stuff gets out-fucked by the bad.
Old One Eye
Old one eye is the only unique Carnifex in the entire Tyranid Codex....along with being one of the few unique characters in said codex. He's known for being ultimately fluffy but not the slightest bit crunchy to back his fluff up.
Old One Eye is a Carnifex that was present when the Tyranid Hive-fleet Behemoth came to fuck the Ultramarines world Calth. Now, like most Carnifex, his armor was nigh impenetrable so one unknown guy ignored his armor by shooting a Plasma bolt through his eye and into his skull, which everyone thought killed the beast and this also gave him the name of Old one eye. He then froze up during winter and left behind after his Hive-fleet left. Being on an Ultramarine world, he inherited the Ultramarines' plot armor, which was plated to him like armor on a Terminator suit.
Old One was then found by some smugglers and thawed out because they though they could sell the giant beast, to their surprise, OOE regenerated and fucked their shit up, ravaging Calth along with the Tyranid remnants from the original invasion. The Ultramarines, unable to stop him with regular means, sent in Scout Sergeant Telion, a man with an equal amount of Plot armor. Telion hunted OOE down and managed to disable him by shooting his ruined eye socket, which caused OOE to stumble into a large ravine and though to have been killed. However, since he is plated with the same plot armor of the Ultramarines, he survived and goes around Calth, ravaging more and more Imperial cities around. Now, this isn't OOE's only plot armored moment, OOE has been hit on numerous occasion that even a regular Carnifex would have died, but it's been rumored that he has been killed dozens of times, only to get back up again like some Necron player with cheating amounts of good luck with his WBB rolls.
Table-top wise, OOE is a unique Heavy support choice who's more expensive than a regular Carnifex (Around the cost of an SM Land Raider), while not being able to significantly outperform a vanilla Carnifex, in the retarded 5th Edition that is when the Tyranid army went to the shitters, much like the space zombies. So yeah, he's going about, being the avatar of what is wrong with the 5th Edition.