8 February 1502
The first thing you must know is that demons cannot, as a general rule, actually purchase human souls in any way that ultimately matters. Whatever happens in the Beyond, no one crosses over led by a demon who has any hold over them. It is enough, however, that humans believe they can purchase souls; as long as one is convinced they are irredeemably damned, the demons seem to get what they want in general.
The second thing you must know is that not all demons have any interest in playing this game, and those that do pride themselves on the difficulty of the 'purchase'. And one demon in particular is widely regarded by his ilk for refusing to pursue anyone but clergy and the most devout of laypeople. This demon, like all his peers, goes by many names. The people of this story knew him as Buné, Duke of Hell.
Buné had taken a certain interest in the Iberian peninsula for its interactions between Christians and Muslims, both in terms of growing wisdom and occasional warfare. During the Granada War, one man caught Buné's attention: a young warrior named Tadzio García. Tadzio was strong and beautiful, recently wed to a woman whose love he'd shared since their youth, unshakably devout in his faith, and skilled with a sword. He proved himself in battle in Granada, and returned to Toledo to enjoy the company of his lover and friends. He was exactly the sort of challenge Buné sought to claim, and he began to bide his time waiting for opportunity to arise. He would not have to wait long.
Tadzio's wife, Ysabel, fell deathly ill. He couldn't bear the prospect of his life without her, and sought help from every avenue he could find. The best doctors in the kingdom could do nothing, and no priest could cast out any evil from her body. When there was nowhere left to turn and she was nearing death, Tadzio locked himself in her chambers to wait out her final days by her side. He took no food with him, and those who loved the couple took vigil the night of his departure and prayed for their souls. It was known that neither intended to leave those chambers alive.
It was here that Buné finally approached him.
The offer was simple. Tadzio could live the rest of his earthly life with his love, alive and fully recovered, if his eternity was handed over to the demon. Tadzio refused, for even in his weakened state his faith remained. Buné left him, only to return six hours later to make the same offer. Tadzio again refused. And so it went, every six hours, for six days, Buné would appear and make his offer, and Tadzio would send him away empty-handed. But Tadzio was growing desperate, and Ysabel was gasping her final breaths when Buné again stood by her bedside and rested his hand on Tadzio's shoulder. Tearfully and reluctantly, Tadzio accepted the offer.
It was three more days before the pair emerged from her chambers, and they were greeted with celebration. Those who loved them received them as though risen from the dead, and a great feast was thrown in their honor. Buné attended, and though he was disguised to all others, Tadzio saw his true face and was reminded of the price he had paid.
Selling oneself to a demon comes with certain expectations of service, and Buné had use for a sword. For a year and a day, Tadzio's sword was used for demonic purposes, and it was growing more and more difficult to hide the truth of his new lifestyle from those around him. Burdened by guilt and dangerously close to being revealed as an agent of darkness, Tadzio finally stole away to a completed portion of the Monastery of San Juan de los Reyes. There, he cried out to God, seeking forgiveness for his sin and escape from his service to Hell. He repented of the holy blood his blade had consumed.
He remained there all night, weeping before the crucifix, before he finally felt absolution wash over him with the morning sun. He rose and found his sword dripping blood, rejecting the work for which it had been used. When he turned to leave, he found himself facing Buné. The demon's feet smoked from the consecrated ground, and its arms were wrapped around Ysabel. Enraged, Buné declared that if Tadzio would break his deal, then he must lose what he had purchased with his soul. With that, the demon slit her throat and dropped her at its feet. Tadzio rushed forward to hold her, weeping and trying to offer some comfort.
"I swear to you," Buné whispered into his ear, "as long as I walk this Earth, you will never join her." With that, the demon vanished. Tadzio was found with a bloody sword and his murdered wife, and along with the growing suspicion surrounding his activities, he was forced to flee the kingdom in disgrace. As he watched the coast of Aragon vanish in the distance from a boat he'd hired in disguise, he swore that he would some day find the means to destroy Buné.