Adult Hellfire Dragon

Physical Description.

"The massive creature is draconic in form, with bone spikes jutting from its head and having intense yellow eyes. Its scales continuously shifts color, creating the appearance of flowing lava. Smoke rises from its rough, spiky scales that are enveloped in black flame. Cruel horns frame its wrathful yet deceptively calculating and cold head. Its body glows faintly with malicious heat, with an overwhelmingly strong smell of brimstone that floods your nostrils as the creature approaches."

Hellfire Dragons. 

Hellfire dragons are powerful otherworldly dragons that have absorbed the powers and essence of the Nine Hells, transforming them into Devil-like dragons. They are corrupt, evil, subtle manipulators and corruptors, seeking to further their own goals and desires by any means. Which often means to grow their influence in the Devil hierarchy. Helfire dragons serve, in some fashion, the devils of the Nine Hells by corrupting and manipulating events among the mortal peoples. When they manage to slip into the material plane, they will often set a secret lair in a region, then use their wealth, knowledge, and skill at persuasion to worm their way into regional politics. Their area of influence typically extends through several kingdoms, and the dragon will have a different Tiefling or devil identity for each realm it is involved in. Naturally, they will go to great lengths to keep their true, draconic nature and form secret. Hellfire dragons will then steer rulers into fighting unjust wars, supporting unholy causes, or even committing despotic acts against the ruler's own people.

Infernal Heritage.

Hellfire dragons hold great power and influence over the Nine Hells. With older hellfire dragons possessing power and status that rivals even that of the archdevils themselves. Hellfire dragons are often leaders of other devils, or are servants of more powerful devils. Though hellfire dragons can ally themselves with most creatures to complete a certain goal, the only creatures that hellfire dragons will never ally themselves with are demons and other hellfire dragons. Possessing an innate hatred against demons, and seeing other hellfire dragons as rivals to be eliminated. This causes hellfire dragons to form grand rivalries that can span many centuries, and therefore causes hellfire dragon matings to be rare. 

Deal Makers.

Hellfire dragons of young age or older have the power to make deals like devils, the power of which depends on what age the dragon is. Young hellfire dragons can make deals like a lesser devil. Adult hellfire dragons can make deals like a greater devil. While ancient hellfire dragons can make deals like an archdevil.

Hellfire.

Hellfire dragons are empowered by and wield an incredibly rare and dangerous power known as hellfire. An unholy black flame that burns both body and soul of those unlucky enough to be scorched by it. The dragon uses this hellfire to claim the mortal souls of those killed by it, sending them directly to the Nine Hells. Hellfire is so powerful and complex in nature that even the oldest hellfire dragon struggles to master its full potential. The only creature known to master hellfire itself is Mephistopheles, the archdevil of Cania, the eighth level of the Nine Hells.

Infernal Connection.

Like most devils, hellfire dragons have an unholy connection to the Nine Hells. Where an hellfire dragon to be slain on the material plane, it would simply rejuvenate in the layer of the Nine Hells it calls it’s home. Only by slaying an hellfire dragon in the Nine Hells could it be slain permanently. While the Nine Hells are the hellfire dragon’s true home, if an opportunity to travel to the material plane where to present itself, most hellfire dragons will gladly take it. Seeking to claim as many souls as possible while on the material realm. This will lead to hellfire dragons attempting to find ways to invade the material plane, where they will fulfill their twisted plots and desires.

A Hellfire Dragon’s Lair

A hellfire dragon keeps and maintains several homes in the cities found within the kingdom the dragon covers. These homes are often large, resplendent mansions guarded by mercenaries hired by the hellfire dragon, who may not even know their master's true nature. These homes are just for show, though, and the hellfire dragon only stays there in its tiefling form when it has tasks it needs to do in that city. When the hellfire dragon is not activly scheming, it retreats to its actual lair, which is either inside the central vent of a volcano, a forgotten ancient ruin, or an underground cavern complex deep underneath the kingdom's capital.

Because the hellfire dragon is often away from its true lair, it never leaves it unguarded. Twisted fire elementals lurk outside the entrances of the lair, while the hellfire dragon's infernal minions, such as barbed devils,  bearded devils, hell hounds, imps, lemures, merregons,  nupperibos, and spined devils, patrol the inner passageways and chambers. Finally, the hellfire dragon's innermost sanctum in its lair, where it hides it precious collections and accumulated treasure, is always protected by magical traps and sealed by magical locks.

Lair Actions

On initiative count 20 (losing initiative ties), the dragon takes a lair action to cause one of the following effects; the dragon can’t use the same effect two rounds in a row:

Hellfire Storm. A storm made up of sheets of roaring flame appears in a location the dragon chooses within 120 feet of it. The area of the storm consists of up to eight 10-foot cubes, which the dragon can arrange as it wishes. Each cube must have at least one face adjacent to the face of another cube. Each creature in the area must make a DC 20 Dexterity saving throw. A creature takes 33 (6d10) fire damage on a failed save, or half as much on a successful one. The fire damages objects in the area and ignites flammable objects that aren't being worn or carried.

Infernal Wall. The dragon creates an opaque wall of fire on a solid surface it can see within 120 feet of it. The wall can be up to 30 feet long, 30 feet high, and 1 foot thick. When the wall appears, each creature within its area must make a DC 20 Dexterity saving throw. On a failed save, a creature takes 27 (6d8) fire damage, or half as much damage on a successful one. Each side of the wall deals 13 (3d8) fire damage to each creature that ends its turn within 10 feet of that or inside the wall. A creature takes the same damage when it enters the wall for the first time on a turn or ends its turn there. The wall disappears when the dragon uses this lair action again or when the dragon dies.

Paralyzing Gaze. The dragon targets a creature it can see within 60 feet of it. The target must succeed on a DC 20 Wisdom saving throw or become paralyzed for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Regional Effects

The region containing a legendary hellfire dragon’s lair is warped by the dragon’s magic, which creates one or more of the following effects:

Hellish Land. Unbearably hot, sulfuric smoke obscures the land within 6 miles of the dragon's lair. Creatures take 3 (1d6) poison damage and 3 (1d6) fire damage at the end of each hour spent in the area.

Eyes of Hellfire. Numerous invisible, small, spectral orbs of black fire resembling eyes stalk about within 1 mile of the lair. The eyes are not sentient on their own, but are controlled by the dragon’s will. The dragon can use an action at any time to see through any of the flaming eyes. The eyes are immune to all damage and succeed all saving throws, but can be dispelled by a successful dispel magic DC 16, or an antimagic field.

Infernal Portals. Rocky fissures within 1 mile of the dragon's lair form portals to the Nine Hells, allowing hell hounds and lesser devils (CR 5 or less) into the world nearby. The portals cannot be removed or destroyed as long as the dragon lives.

Sulfuric Water. Water sources within 3 miles of the lair are supernaturally warm and tainted by sulfur.

If the dragon dies, these effects fade over the course of 1d10 days.