Nearly one thousand years ago, in order to save his family from the Gurongi, a father found a way to give his family immortality. They became the first of the Fangires, able to survive and fight. However, they also had abilities that the Fangires they would sire would never have.
As long as the Original Fangires would absorb life energy, they could function as any other Fangire. However, they are effectively immortal as long as they perform a certain ritual every month, involving a number of corpses and blood. It draws a lot of attention, that's the downside.
The Original and normal Fangires retain their human age as they live. Normal Fangires will die if killed, but killing an Original only slows them down for some time until they recover. They can be killed for good by using the Zanvat Sword on them.
Their father has turned against his children by this point, seeing the destruction they cause. He wishes to purge them, and is trying to recruit a number of them to his side out of fear, waiting for his chance to kill them all.
Father: Dark Kiva/Fangire King Daughter: Kivaala Sons: Kiva/New Kiva/Saga/Arc
Saga/Arc could be a woman. The wife/mother is also possible.
Dark Kiva: Me Kivaala: New Kiva: Me Kiva: Rei Saga: Arc: