King Harvald's Bargain

Perhaps the most widely known contract with the lords of Hell, King Harvald's Bargain was meant to aide the high king of Hoabb in winning his war with the Alliance of The Bite to the east, but ended with his people being transformed into hybrid hellspawn called goblinoids.
After being signed in royal blood by High King Harvald, the rightful ruler of all lands and peoples bound to the title of the high king, the bargain was finalized when the new moons reached their zenith and a small rift to Hell opened in Hoabb. Instead of fiendish warriors come to supplment his troops, a torrent of unbound fiendish spirits poured through flooding the streets of the capital Hitan Wilps with screams. Far from the capital, across not just the lands of Hoabb but everywhere a person owing allegiance to the high king was located the spirits sped, being blocked from their victims by only strongly sanctified sites or individuals magically protected from fiendish domination.
After the souls of the people of Hoabb were fused with the spirits, a few fortunate individuals were saved by priests and clerics of other nations. Most of those saved were captives of the Alliance of The Bite, either in lands of Hoabb held by the enemy, or those held for ransom in one of the Gold Coast kingdoms. Unfortunately for those not promtly attended to, the goblinoid transformation could not be reversed after the moons had risen again the next night.
The nobles and aristocrats of Hoabb had been transformed into Hoabb Goblins, or hobgoblins, a clever and organized but decidedly evil and sadistic subspecies. Among the hobgoblins traces of the personality of the victim and most of their memory remained intact. Knights and citizen warriors were transformed into bugbears, often having their soul sundered into two fragments, boosting the numbers of Hoabb's armies shock troops and captains. The serfs and peasants of the land were reduced to varied forms of minor goblins, having their souls shattered into two or more pieces, giving rise to teeming masses of goblin levies for the king's armies.
After the shock and terror of the arrival of Hoabb's goblins, the lands of Hoabb were all recaptured by its new troops, and the smaller holdings between the Alliace and Hoabb were temporarily overrun, with most of their citizens captured and settlements razed. The Bargain was fulfilled and the effects permanent. Many generations later, the goblins show little hint of their first world roots in humanity, except that they suffer both from the touch of cold iron as a fiend as well as the banes of their first world kin.
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Originally written for 2024 Summer Camp challenge, answering Document A DOCUMENT THAT CHANGED THE COURSE OF HISTORY