The Legacy of Kain is a videogame series developed by Silicon Knights and Crystal Dynamics. The series starts with Kain, a spoiled young noble, assaulted by bandits and unwillingly transformed into a vampire. He would eventually choose to lead the entire world to ruin in order to have the chance to possibly, maybe, set things right and protect the world of Nosgoth from acient monstrosities. Time travel factors heavily into most of the installments, although the characters soon learn that they are not truly changing the past; they are just replaying events which have already been determined.
Ariel was the final human Guardian of Balance, murdered out of jealousy. Her death marked the start of the Pillars' corruption, and her grief bound her soul to the Pillars. She lead Kain, the new Guardian of Balance, on a quest to slay her corrupted comrades, then urged him to kill himself to remove the corruption. Doing so would have also marked the extinction of the vamprie race, however, and it was the immortal vampires who were the intended Guardians of the Pillars.
Kain shuns her then, leaving her soul in unrest. She remains bound to the Pillars for thousands of years, until she finds an ally in her quest to slay Kain in Raziel. He follows her guidance for a time, until he learns that perhaps Kain's way was for the best. Her soul finally recieves rest after she, during the time of Blood Omen, purifies the Reaver for Raziel.
The Hylden are a race as ancient as the vampires, and in times long past waged war against the vampires. That war ended with the vampires cursed with a bloodthirst and the Hylden banished to an alternate dimension. While banished they manipulated events to destroy the pillars and drive vampires to extinction so that they could eventually return to Nosgoth. The time in the alternate dimension left them twisted and deformed, however, and little more than monsters.
In the cancelled Dark Prophecy the original, un-corrupted Hylden could have made an apperance.
After founding his vampiric empire Kain breathed life into six "sons", who in turn made their own vampiric underlings. After Raziel was cast into the pit his clan was wiped out by his remaining brothers, leaving not a one of them alive by the time it took Raziel to return to Nosgoth. In the cancelled Dark Prophecy members of Raziel's clan might have made an apperance.