Sinh Mourn was an Imperfect Yokai whom was a widespread menace well before his ascent to nearly humanoid form; the avian firefiend manifested blades of sheer flame in combat since its early Kaor stages, practicing a strange discipline of martial swordsmanship said to linger in some parts for its brute efficacy. He was a powerful force amongst both the Empire's armies and the Seers of Darkness, participating in the entirety of their conquest over Valmasia in the shadow of the dreaded Kokb'ael Kzer-za.
Sinh Mourn's beginnings go back to the Yokai woodlands, in the advent of Eline Sakete's own empire in which the political climate of the world became more individualistic and diverse for Valmasia's prominent villages. In his beginnings he knew many rivalries and sought to prey on all in the forest - but after falling as a Giga to a lesser evolved Demergo Veltican, the avian Yokai retreated from his ventures to a life of scavenging and subsistence as he healed from his wounds.
Decades later the Yokai reappeared amongst Eva's forces; he was initially just another brute minion of the citadel's unholy mistress, but evolved rapidly and developed an embittered hate of the Kaor herself, even as she ascended to the status of Imperfection. She was separated from her own followers and seldom joined their hunts herself- Mourn himself, as a Kaor, led her flock in her own stead across the forests and plains of Valmasia in butchering raids and manhunts for flesh. With his own followers and Eva's, he killed many and slew dozens of Sacred Knights, developing a hatred for the Urayans as they hunted and fell to him in turn.
As the forces of Byson grew thicker near the beginnings of the Seers of Darkness and their return prior to the rise of Aria Hirano's Empire, the eastern city's army prepared to overwhelm the citadel and expunge the forces of Eva, wary of her rise to Imperfection by the dark echo of her Occult presence. Unwilling to lose her citadel or to allow the East to overrun her fortress, she sent her armies to attack Byson instead, drawing on her unsavory allies and sending Mourn to lead her forces ahead: she promised that, with her own powers as an indomitable Imperfect, victory could be had over the Krausite bastion and a feast unlike any other awaited her minions.
However... Eva the Imperfect never arrived to her own war.
In the absence of Eva, Mourn led her Yokai and human slaves, waging as best a war he could together with his closer allies: Darac'tungue the Aquarius, Malachite the Ventus and Desolus the Terrus were among his trusted allies for the siege. With the Avatar of War born of the Mimic's own insipid forces born at the dawn of their creation they fought a very viable attempt on Byson's mainland- but without Eva the Imperfect herself, the forces of the Yokai were ultimately defeated and Mourn himself fell in battle, though he survived and was carried away by a pack of slavering lesser Yokai.
Mourn later abandoned Eva and led her own followers away from her; they traveled abound, marauding just out of any militia's reach, managing to defeat even the Champion Anariel Melym and her sword Almace, though the powers of the Aether Cloak allowed her to freely escape an unsavory end after falling to the powerful cadre of rogue Yokai. Later, when the Seers of Darkness extended official invitation abound and came to public face as a blatant power, Mourn himself joined their ranks though his allies abstained; Malachite, a Yokai Mourn himself defended and raised since their lesser stages, held outright suspicion of the cult for her own interests.
However- alongside Kokb'ael Kzer-za and his own allies, Mourn brought his fiery bladesmanship together with the equally auspicious combat prowess of his own companion Kaor against the forces of Danarium and their coalition, helping the budding Empire to its first victory.
Elated, Mourn traveled Empire territory freely and even battled the Vindicator Korrin Sakete for rights to land in Alteros, winning his right to Alterosi soil by the terms of a duel. There, he made a battered old house the seat of his power, spawning a small horde of infernal impish Yokai within and dueling powerful heroes from across Valmasia freely there on his land; he defeated Achilles S. Avharain, Kane Hirano, Aria Hirano herself and numerous adversaries both human and not in auspicious displays of combat. Mourn developed a unique fighting style dependent on blade-like conjurations of flame and sheer brawn used well in swift, punctual combat, reckless at worst but terrifying at best.
He acquired the Cursed Sword Durendal from Eva, which he used for years to supplement his powers, refine his swordsmanship further and temper his malice to greater heights of inhuman killing intent vital to the core of his ruthless sword style. Its eerie whispers and mind-twisting, soul-rotting influence destabilized the Kaor however and tormented him; no less, he mastered the legendary blade and even defeated his age-old rival, Demergo Veltican, utilizing its insipid powers and his own overwhelming fury stoked by the demonizing sword.
Thereafter he fought in a short rivalry with Ulfdyr the Fair, whom defeated the Kaor using the powers of Wrath and his enchanted Cold Iron armor, using Mourn's blood to forge the cursed hammer Gotterdammerung. In addition to the corruption of Durendal, later taken from him by a rogue Antoinette Avharain after her onset of madness following her self-grafting of a Malpercian arm, the influence of the Sin of Wrath impacted Mourn to severely alter his psyche: his hatred of humanity and designs of warmongering grandeur were forever multiplied, adding the finishing touches to his relentless warrior mentality and misanthropy.
After the first Empire war, Mourn was made a more inclusive member to the Seers of Darkness, and joined in the feasting of countless captured prisoners and civilians sacrificed upon runic circles to hasten the Perfection of Kokb'ael. After consuming his own share of the fleshy banquet, and witnessing Kzer-za's true form of skeletal horror and unearthly shade, Mourn abruptly ascended in a volcanic shudder that cast ash abound over Valmasia to arise higher as Sinh Mourn, the Imperfect.
Sinh Mourn ventured with the Empire and fought in all their wars; he retrieved the lost sword Auroralie from a wayward warrior, tainting the weapon in Glacium blood as he wielded the fabled blade against innocents and brave soldiers, committing acts of heinous murder and ferocity with it alongside his own Kaor Blade. Defeating the champions of Frostvale, the legions of the Glacium and later even multitudes of Urayan forces seated in the Wanderer's Valley, Sinh Mourn quickly became an influence of terror and fearsome might on Valmasia; even after selling years of his life to the time-warping sword Chronos, mastering the magic of the Reversed Fears and producing his own advanced discipline of Rift-Magic Swordsmanship, Achilles S. Avharain- one of the era's most powerful human warriors whom carried significant momentum against the Empire- could not defeat Sinh Mourn or his own bizarre style of hellfire and culling steel.
After seeing the Empire to victory, Sinh Mourn embattled Kokb'ael Kzer-za himself, wounding the near-Perfect... but not to the measure of maiming, and a single series of electrifying blows imbued with nearly divine Occultism defeated the ascended firefiend handily. In his recovery he was no less feared however- and was a vital force, alongside Sareen the Mimic and Vindicator Qaelthan- in defeating the Angel Kushiel, Vindicator Tulise as well as the divine-resurrected Demergo Veltican in the coup for Alteros, placing it under Empire control while it had once been an anti-empire force.
Mourn's life was soon to end and his sword was a grievous obstacle to any threat on the Empire's power; following the coup of Alteros, he was attacked together by Malachite and Demergo Veltican, both shapeshifting Kaor and failed Imperfects themselves; the latter, though not a true Imperfect Yokai himself, held the divine scythe Damocles and the blessings of the Fallen Angel Kushiel which rendered it impossible for Mourn to dismiss the Terrus, enabling him to overwhelm Mourn together with Malachite's own traitorous aid. With his blade gone in the hands of Sareen and his allies content to abide the quarrel, Sinh resolved to drastic ends rather than allow himself to be humiliated or captured by his enemies:
In a roaring incendiary flash, Sinh detonated the entirety of his pyromantic powers, shuddering the eastern plains with a deafening volcanic eruption that hurled magma and flame about in a disastrous spew that choked many travelers in pyroclastic ash and nearly threatened Nostvale's own walls. The suicidal exploson was Mourn's last act, ending his corporeal time on Valmasia as a violent conclusion befitting his impetuous rampage of a life.
Thereafter his blade was taken, then sealed by the powers of Mercy by none other than Anariel Melym herself, ensuring the ascended Infernus could not return to Valmasia without divine aid; he remains bound to the ruins of his Kaor blade today, which has been reshaped to a fiery claw-like gauntlet of cursed powers despite the seal on his soul, awaiting one to succumb to his influence and release the devilish firefiend to pursue his ventures of Wrath and warfare once again.
Sinh Mourn initially took a radical stance among the Yokai of his time: he would not abide human slaves nor condone consorting with their ilk beyond absolute, immediate need.
As a Kaor he fought explicitly to subjugate humanity and reduce them to the corners of Valmasia, to be hunted and terrorized, with lives as short-lived livestock as the most comfortable existence their species would ever again know. He endeavored for a kingdom of Yokai instead- ruled by Perfection and power- to the ends of which he took the prodigial and tragically failed Ventus Malachite herself under his wing, whom possessed keen intellect and devastating magical prowess unique to her talons.
Mourn was once vaguely charismatic and able to sway his fellow Yokai to follow him even as dissonance and brawls murmured between them, born of personal mistrust and bloody brawls... but after the influence of Wrath and Durendal, he was governed solely by battle-lust and self aggrandizement. He left all behind in pursuing his own greatness - which, ultimately, left him alone and defeated in the end.
As a young Yokai he was a beaten scavenger, so his rise to Imperfection from having once been a scrawny minion was a boon to him he nor any of his former peers ever foresaw; Demergo, his once-rival and eventual murderer, held envious eyes for the ascended Infernus' Imperfection where the angel-blessed Terrus himself had failed. Deep seated humiliation, hunger pains and sheer hate gave Mourn insatiable desires for fame, power and sustenance, such that he was able to ascend to Imperfection and ravenously pursue greater and greater heights of immolating might.
Though he abided humanity far more as an Imperfect- partly for their necessary association in the Seers- he remained hesitant, cautious and abstinent of nearly all things humanesque. This sentiment did not hold however as he succumbed to praise and pleasure offered to him by mortal subjects and allies, whom he eventually accepted if only for what they offered him; in time, Mourn came to change his design on humanity, desiring to produce demi-Yokai humans that would lord over the humans of old, acting as an intermediary between the Yokai and humanity as he steadily became aware of human merits, many of which he loathed and feared- but some of which he used and weaponized, amidst his new allies.