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.
Carnifex in the 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In the 5th edition, the carnifex lost a total of eighteen biomorphs and weapon options, having been replaced by more stringent mandatory loadouts.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 only tepid stat increases, putting it arguably somewhere between 20-30 points too expensive for a competitive army list. Since the carnifex was previously one of the Tyranid's most popular and best selling models, there was a wide margin of Tyranid players on /tg/ who called foul on Games Workshop, accusing them of wrecking the one creature EVERY Tyranid player owned to force everybody to go out and buy new models. However, 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, is a matter of rampant speculation though it's probably the former since it's widely known that GW is bleeding money though the third may also have something to do with it since Robin Cruddace isn't exactly the biggest fan of the Tyranids.
Furthermore, the old tournament favorite, the "sniperfex," which simultaenously used barbed stranglers and venom cannons to relentlessly stun tanks into submission, has been completely invalidated. It, like so many other choices, including tusks, spinebanks, scythe tails, extended caraprace, and more, have been stricken from play. But on the bright side, 3rd edition's Old One Eye is back, even if he does now cost more than a Landraider and is barely any better than a normal carnifex with his 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.)
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, 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 played 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 morment, 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.
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.