(From Mordenkainen's Tome of Foes - 2018):

Hellfire engines are semiautonomous bringers of destruction. Amnizus and other devilish generals hold them in reserve until they are needed to repel an incursion by demons or crusading mortals, but occasionally one of these mechanical and magical hybrids gets loose, driven berserk by its need to destroy.

Many Forms, One Purpose. Hellfire engines take many forms, but all of them have one purpose: to mow down foes in waves. They are incapable of subtlety or trickery, but their destructive capability is immense.

Soul Trapping. Mortal creatures slain by hellfire engines are doomed to join the infernal legions in mere hours unless powerful magic-wielders intervene on their behalf. The archdukes would like nothing better than to modify this magic so it works against demons, too, but that discovery has eluded them so far.

Constructed Nature. A hellfire engine doesn’t require air, food, drink, or sleep.

(From Fiendish Codex II: Tyrants of the Nine Hells - 2006):

This automaton thunders forward, its metal plates shrieking with each step, looking like an armored giant. Through the gaps at its joints you can see the white-hot light of some inferno raging within. Its head is terrifying, wrought to appear like a bearded man with the horns of a stag, its mouth opened impossibly wide.

Hellfire engines are infernal constructs constructed as artillery for use in the Blood War. 

Strategies and Tactics: A hellfire engine responds to the simple commands of its creator, just like a golem, although it can also be ordered to follow the commands of another creature. When ordered to fight, it opens up with its breath weapon and then ambles forward, its fists leaving fiery arcs in the air.  

A hellfire engine’s most notable trait is the pure hellfire it spits at its opponents. No creature in the known cosmos can resist a hellfire engine’s awful attacks. Many devils speculate that hellfire engines serve not only to annihilate legions of devils, but also to deter any of the dukes of Cania from turning against Mephistopheles.

Cold Iron Body (Ex) A hellfire engine is constructed entirely out of cold iron. Its slam attacks count as cold iron for the purpose of overcoming damage reduction. 

Death Throes (Ex) When destroyed, a hellfire engine explodes in a 60-foot-radius burst that deals 20d10 points of damage to everything in the area (Reflex DC 32 half). The save DC is Constitution-based. 

Hellfire Shield (Su) A hellfire engine’s body casts of tremendous amounts of energy. Any creature that strikes or touches a hellfire engine with an armed or unarmed melee attack, or that grapples a hellfire engine, automatically takes 6d6 points of damage. A creature takes damage from this ability only once per turn. 

Hellfire Slams (Su) A hellfire engine deals an extra 6d6 points of hellfire damage with its slam attacks.

Hellfire:

Hellfire is the creation of Mephistopheles, archduke of Cania. Hotter than the hottest flames of any world, hellfire burns with a white-hot glow and is capable of burning through even the hardest of substances. Hellfire does not deal fire damage, despite its flames. Even creatures with immunity or resistance to fire take full normal damage from these hellish flames. Hellfire also deals full damage to objects, unlike normal fire damage.

Sample Encounters: Hellfire engines are blessedly rare. The resources required to create them are scarce and only a master of hellfire can construct one. They are encountered individually, though squads of steel devils, orthons, and bearded devils sometimes support them. 

Individual (EL 19): Individual hellfire engines serve as guardians for powerful devils.  

Strike Force (EL 19): This group consists of a hellfire engine accompanied by a squad of either ten bearded devils, six orthons, or six steel devils. 

Ecology: Hellfire engines are tools of war. They anchor armies, lending impressive artillery to the ranks of devils. When not on loan to stem the flood of demon invaders, these constructs serve their creator, Mephistopheles, acting as guards or even just symbols of the devil’s power and prestige. 

Environment: Hellfire engines are native to the Nine Hells of Baator, but they can be found in any land or underground. 

Typical Physical Characteristics: A hellfire engine stands 16 feet tall and weighs 40,000 pounds. 

Construction: A hellfi re engine is assembled from 40,000 pounds of cold iron bathed in the blood of a dozen celestials and then polished with a rare blend of brimstone, sulfur, and acids found only in Hell, costing at least 80,000 gp. Assembling the body requires a DC 20 Craft (armorsmithing) check or a DC 20 Craft (weaponsmithing) check. 

CL 25th; Craft Construct (MM 303), hellfi re storm, geas/quest, limited wish, polymorph any object, caster must be at least 25th level and lawful evil; Price 320,000 gp; Cost 200,000 gp + 9,600 XP.

Hellfire Engine Lore:

Characters with ranks in Knowledge (arcana) or Knowledge (the planes) can research hellfire engines to learn more about them. When a character succeeds on a skill check, the following lore is revealed, including the information from lower DCs.

Knowledge (Arcana):

DC     Result

29:     This automaton is a hellfire engine, a construct created in the Nine Hells.

34:     A hellfire engine spews gouts of hellfire, an energy unlike other forms of fire. These creatures are also quite hot, and their attacks burn not only the flesh but the soul.

39:     As creatures of fire and steel, hellfire engines are susceptible to cold. They have a tendency to explode when they are destroyed.

Knowledge (the Planes):

DC    Result

29:    Mephistopheles is the undisputed master of hellfire. He is responsible for a number of abominations tied to this substance, but few are as as dangerous as the hellfire engine.

Typical Treasure: Hellfire engines do not collect treasure, and such specialized weapons of war are rarely used simply to guard a vault or hoard.