Alexander was a Hungarian warlord who lived in the early fifth century. He ascended to power just in time to watch his village become ravaged by an unknown plague. In the end, Alexander was the only survivor of the plague. His body had in fact adapted to the virus, and through some unknown means it changed him. It granted him great strength, stamina, and longevity. He could not die of old age and he was impervious to all diseases. In essence, it made him the first true immortal.
Birth of the Underworld:
Years later, with his wife Helena, he fathered three children, two of whom inherited the immortal strain in its active form: twin brothers. Their names were Markus and William. Both sons were bitten by animals later on which seemed to have a physical effect on them. Markus was bitten by a bat and William was bitten by a wolf.
After those events, Marcus went on to become the first vampire; whereas William became the first lycan. Marcus possessed a unique control over his ability. He needed blood in order to survive and become stronger, but he kept his thirst in check. Marcus' only weakness was to sunlight.
William, on the other hand, was transformed into a lycan. Unable to change back, into a his human form, William became like an animal. He lacked the control that Markus displayed. Instead, William developed a strong appetite for carnage and destruction - one that could not be controlled. Though he possessed great brute strength and stamina, he lacked the focus needed to keep himself in check. Thus, William was lost.
Each brother had the ability to pass on their unique "gifts" to mortals. Through those means, both races began to multiply and eventually, a war ensued. The vampires sought to control William's appetite for carnage and eventually imprison him for life. Throughout all the bloodshed, Alexander Corvinus remained in the shadows. Through his gift of life, he had brought about so much death. He knew that William needed to be controlled, but he did not have the heart to kill his own son.
A New Beginning:
Hundreds of years later, Markus awakened from his long sleep as a new hybrid species. Because he was much stronger, Markus felt as if he could control William. However, Alexander knew that it would be impossible. If William was released, then there would be no way to control him and millions of innocent people would lose their lives.
Markus felt as if the gift of immortality should not be kept in the shadows. He wanted to rule the world alongside his brother. Eventually, Markus came to think of himself as a god. Pent on world domination, he began creating a way to unleash his brother. In the process, he confronted Alexander one last time. Markus then stated that he was a god, and that a god has no father. With those words, he plunged a sword into Alexander's heart.
Before he died, Alexander ensured the safety of the world. He did not have the heart to kill either one of his sons; but that did not mean that someone else could not. Thus, he contacted a vampire named Selene. He allowed her to feed from his blood as he was dying. This enabled her to gain the strength she needed to defeat Markus and save humanity.