Nethereal Inferno

Nethereal Inferno alternatively referred to as the Raging Hellfire of the Satan is an incredibly powerful Sacred Gear said to contain the soul or spirit of none other than the most powerful of the original Four Great Satans, the original Lucifer, a power said to make it the natural opposite of the True Longinus, a Sacred Gear reputed to hold the power of God. One of the biggest bugs in the Sacred Gear system for sure, this power is in fact not called a Longinus due to it never being in the System of Heaven to begin with, and is a power absolutely no-one has proven to know for sure why it even exists, as the most influential Devil in the Underworld's history somehow managed to slip into the Sacred Gear System. A bug on a level that can no longer be called a Longinus, this power is instead called a Sigil, a power that overshadows the Longinus as the biggest bugs not because of only power, but by what the Sacred Gear itself actually entails, and is representative of the Devils of old slowly mainfesting their will on the struggliing Sacred Gear system.

Summary
Nethereal Inferno appears to be the beginning of Three Impure Symbols, and is both the Fires of Hell and the Lake of Fire often associated  with the power of the Underworld. The Nethereal Inferno does not need a host to operate, Lucifer's will driving it forward, though it only fully harnesses itself to it's fullest potential when under the will of a host.

Abilties
While many, including the extremely powerful Evil Dragon Vritra, as well as God himself, ahve tried to replicate and/or surpass the natural power of the Underworld, ther Nethereal Inferno is in fact the most powerful flame in existence, atleast known as up to this date. Composed of the very Chthonian element said tod rain life away from all living things, the destructive aspect of reality, this power is incredibly powerful, and is perhpas the most concentrated application of demonic power ever seen in an extremely long period of time. Nethereal Inferno has the power to emit truly vast, almost limitless amounts of potent blue flames representative of the truest materialization of hell-fire, capable of completely burning even Devils incredibly easily and works with the power and effciency of several Demon Swords on Angels and Fallen Angels, greatly harming them upon impact and slaying even the most powerful of that kind with ease, including even Seraphs, a power similar to Incinerate Anthem, and is said to powerful enough to slay Gods potentially in the hands of a master.

Possessing an incredibly powerful near solar level heat, if this heat was then condensed into the heat in a singular particle of the plasma composing the blaze, the fire is incredibly, nearly impossibly hot, a heat that it's host can bear without even budging slightly, the sheer heat of the fire said to be so incredibly vast that the heat is considered unrivaled by anything naturally existing, said to be atleast 30 times hotter than even an incredibly powerful bolt of lightning, which by itself was five times hotter than the very surface of the Sun, a heat said to cause the very Earth itself to dry up and it's waters to move themselves in preparation for the final arrival of the adversary, a total temperature which gives us the grand total of being a massive one-hundred and fifty times the heat of the surface of the sun, and this is when wielded without a master of the power. However, at the beginning stage, one's flames are only ten times hotter than lightning, but as one progresses, it can potentially even surpass the 150 times hotter base temperature of an advanced user. Hellfire, or true hellfire, constantly saps energy from it's target and burns the enemy, through burning the life-force of the enemy in question, draining their life energies and stamina over the time if they can even come close to accepting the physical burn to begin with, a fire said to be so potent it burns ash itself. This also makes it incredibly useful against opponents with regenerative abilities, as it greatly slows or even completely halts their regeneration, their endless stamina and life-force being attacked and drained to cause the Inferno to blaze even hottter and greater than ever before.

As the Blaze Black Flare itself states, it's curses are "like" hellfire. This power enables one to produce an even brighter blue flame, which shows the ability to severely damage whatever it comes into contact with, as well as cause one's soul to ragdoll within itself, and if the person is killed with the blaze, drags the soul out and incinerates it, before becoming even brighter, as well as curses things which cannot usually be burned to be incinerated by it's power regardless, even granting itself the capabiltiy to burn for seven days and seven nights unless the fire is consciously cut off by it's wielder, and if the wielder is killed, burn indefinitely unless the next wielder comes along and shuts it down. The good thing about this Sacred Gear is that it gives the wielder an immense amount of demonic power despite not being a demon.

Balance Breaker: Soul Anchor
By utilizing it's ability to assimilate souls, a user of the Nethereal Inferno can activate their Balance Breaker to generate an incredible feat, forming the shape of the flames directly into the form of absolutely anything the flames have ever burned and eliminated before, whether physical or non-physical, animate or inanimate. This incredible power allows one to bind souls to the flames, creating a vast amount of servants out of the fire at will, or like the power of the Incinerate Anthem, that activate themselves without the user even thinking about them. However, where the Inferno shines in this is in that being the true hellfire, the Inferno has shown capable of burning absolutely anything sent to the Underworld after death, gaining their souls for the taking. As a result, a master of the Balance Breaker can atleast summon at bare minimum two souls and compose them completely out of fire, as well as the souls of all those who accept the Evil Pieces system, as their human souls are now the Sacred Gear wielders to emulate indefinitely, capable of bringing forth powerful Sacred Gear wiellders to assist the user in battle, as well as utilize the power his former foes slain by the flames against his other opponents. However, it can also allow the user to harness the power of and manipilate the souls of the damned if pressured enough.

Subspecies Balance Breaker: Dominance of Hell
However, Seiko, the current known wielder of the power, has harnessed his Balance Breaker to produce an entirely different result, awakening the power of all of the many alterations of the Flames of Hell and the Underworld, vastly augmenting his own power, in addition to gaining control to impose the will of Hell on almost any flame, as well as the power to assimilate any flame beneath the Inferno's power and add it to the Blazes of Hell. In detail, this power grants Seiko the ability to control almost all forms of the fire element in general, allowing him to take command of a small blaze and force it to attack his opponents, and even shift the form of fire it is from any one of the seven types of hellfire, to a random release of regular fire, or potentially even the flames of a Dragon. Apart from that, the user can assimilate any new supernatural flame that they come into contact with and blend it with their own, as well as seemingly consume the flames before them with no harm, consuming all of the fire easily.
 * Almighty Blaze of Lucifer: All Powers before this point
 * The Blaze of Limbo (Belphegor): Seiko is granted the ability nullifying blaze of the Seven Deadly Sin of Sloth, Belphegor, the first sign of the powers of this Sacred Gear being akin to the power of the True Longinus. This allows him to burn and destroy the very essence of special abilities for as long as the flame is active, as well as apply ti on him to help get rid of any curses, etc, though it does not burn physically. It also has the power to curse the opponent with sloth and a deduction in their speed.
 * The Argentous Phantom Flame of Hades (Leviathan) :Leviathan of Envy, Seiko can absorb all the heat from the area  and turn it into ice as a silver flame, as well as absorbing the heat and thus to an extent the kinetic energy behind the attack as kinetic and thermal energy are in actuality the same thing.  It also grants Seiko thermal vision. Inversely, it can infuse incredible levels of kinetic energy to whatever it burns, causing implosions to non living things.
 * The Dichromatic Twin Flame of the Netherworld (Mammon): The power of Mammon grants the user the ability to create incredible powerful whirlwinds of unimaginable temperature, and allows them to achieve many feats such as controlled flight, as well as the manipulation and levitation of objects around him through the manipulation of surrounding air pressure. It is rumoured that this flame's true power is to burn space and time, causing gaps in them which allow the user to teleport upon the dispersal of the fire.
 * The Achromatic Catharsis Flame of Purgatory (Beelzebub):  Seiko can manipulate an incredibly potent invisible fire even hotter than Lucifer's blaze, capable of vastly enhancing Seiko's striking power, creating an invisible armour out of it. With this fire, Seiko can create invisible arms made out if this blaze to attack his opponent mid-fight, as well as even an incredibly hot invisible blade in combat, or in fact, even raining the fire down as if it came from the clouds for incredible destructive capability.
 * The Iridescent Explosive Flame of Hell (Asmodeus): Seiko is capable of utilizing a blaze that creates an incredible lustful illusion capable of disarming his foes and or trapping them in an illusion that makes use of all five senses to trap the target in their deepest, darkest, sexual fantasies, whether repressed or not, and is known for even creating such fantasies in asexuals, while slowly consumiing them, the rate being up to Seiko. However, apart from that, it can create massive explosion of iridescent colour so powerful it can destroy the inner ears of those with sensitive hearing.

Weaknesses
With the power of a Devil locked inside, one becomes more vulnerable to weapons and energies of a holy or exorcist-type nature, however, due to having the power of some of the greatest Devils, the original Satan-Class seven deep within it, the user's raw demonic power even as a beginner ensures that they are not one-shotted by light some random scrub, taking a good deal of holy attacks to take advantage of their vulnerability, though one may still be injured because they are simply not durable enough, taking the normal damage they would take instead of damage + weakness.

Trivia

 * Not all powers here have been awakened by the Sacred Gear's wielder as yet.