Many years ago in the middle of a vast countryside, there lived a rich nobleman and his two sons. After the death of his beloved wife at the hands of a demon, the man took up his sword and waged a personal war against the plague of evil that haunted mankind. As his sons grew older, he attempted to teach them the way of the Hunter. The elder of his sons possessed an amazing aptitude for hunting, a natural instinct that served to get him out of whatever dangerous situations he had managed to get himself into. The younger son, while possessing similar instincts, was more inclined to the pursuit of knowledge as opposed to that of vengeance. He would stay behind and research in their vast library while his father and brother hunted all manner of evil creatures.
Four years after the death of his wife, the nobleman's mistress died after giving birth to a son. In spite of his quest for vengeance, the Hunter was a good man. He took the boy into his home and declared him both a brother to his boys as well as an heir to his estate. Both the father and his sons sheltered the new addition to the family as much as possible from the reality of the Hunters' world, but it couldn't last forever.
After fifteen years of tracking the son of a bitch demon that had killed his wife, the Hunter's story ended when he took a blow that was meant for his eldest. Enraged by the death of his father, the Hunter's son managed to throw a special demon-killing dagger, forged by the legendary Samuel Colt. The dagger pierced the demon between the eyes, killing him instantly. Grief-stricken, the son brought his father's body back to the manor to be salted and burned, as was traditional in the Hunter's world, and then all three brothers settled into their particular brands of mourning.
Unbeknownst to them, the demon (whose true name was Azazel, but that is neither here nor there) had a daughter named Meg. She took offense to the death of her father and decided to exact revenge upon the house of the fallen Hunter, his eldest son in particular. Knowing that he had a penchant for drunken one-night stands, Meg seduced him and plied him with alcohol before convincing him to take her back to his manor. The minute he allowed her past the anti-demon warding at the front gate, Meg immediately knocked the unsuspecting Hunter unconscious and began to recite her curse.
By the time he regained consciousness and had awareness enough to do anything about his situation, it was too late. The girl he had returned with the previous night stood over him, one foot planted on his chest, her eyes as black as the pit she had dragged herself out of. A thrill of fear traced itself through the Hunter's frozen form as she spoke the final words of her curse.
"And so you shall be a Beast, hideous to all who behold you, until the day comes when an Angel raises you from this Hell by agreeing to be bound to you in love and not by force. Until that day, the House of Winchester shall be held in time by this curse." With a cackle, the demon departed, her final words echoing through Dean Winchester's mind before being chased away by his own pained screams as he transformed.