From Exterminatus Manual
Tyranid Carnifex (Fex)
Carnifexes are living engines of destruction. With a thick carapace that can soak up damage, talons that can rend through armor and a lethal plasma scream a carnifex is the pinnacle of the Tyranind classes.
Carnifexes are armed with a combination of Bio Plasma for medium/short range and Scything Talons for close range attacks. Primary attack fires the Bio Plasma and secondary attack lashes with the Scything Talons.
Carnifexes have the greatest durability of any class in Exterminatus, however their armor is more effective against weaker weapons (bolt pistols will not penetrate carnifex armor at all), you still need to respect weapons like the missile launcher at range, and the melta gun in close quaters. See Bio Plasma and Vennon Cannon for tactics relating to the wargear.