To understand Oporo, Rapithwin, and Saturna, it's easier to explain them all at once.

Oporo

Rapithwin

Saturna

Rapithwin was banished from the gods because she was attempting to make and perfect a weapon that could destroy them. Her experiments had to do with separating souls from bodies. Through this process she began to discover that souls held tremendous potential energy and could be used to make a weapon.

Upon her arrival on earth she was split into three portions, each part of her soul separated into an identical body ("bad" Rapithwin: Saturna, "good" Rapithwin: Oporo, and mid Rapithwin: just Rapithwin). She kept her separations a secret from everyone else. She (meaning all three women) then began applying her techniques to humans which resulted in making vampires (by removing their souls) and zombies (by injecting those souls into dead bodies). The vampires did not live long (she didn't yet realize they needed blood). The zombies were lower functioning than she would have liked and craved flesh. She finally discovered that vampires needed blood to sustain their lives.

While this was going on she was posing as one person, only one of them going out in public at a time, or being seen by any of their creations. She (all three) fell in love with Edgar, a potential subject. She waited until she had figured out the blood trick before turning Edgar into a vampire. He is the only creature aware of her secret (though now he's dead, so it doesn't really matter)

Lots of fighting ensued with the Rapithwins over what was appropriate (don't kill to live, kill anything to live, do only what you need to live). Edgar eventually sided with Rapithwin because she had the middle ground approach. They decided to leave the mainland together and found the island to live on. Edgar wasn't the only breaking point between the Rapithwins. The use of the souls they had collected, and other things going on between them had all added up to the dissension and eventual separation of the three.

Rapithwin hid her ultimate weapon at the bottom of the lake. She had to kill the limnatide in order to do so (a terrible necessity). She had to erase the memories of the Vatnadis to assuage their grief and anger. She adopted them in a way, along with the small faction of humans and other magical tribes that had been living peacefully there. She took on the safety and well-being of the island as her responsibility.

While Rapithwin was on the island Saturna attempted to kill Oporo. She was still heartbroken over the loss of Edgar and took it out on the other part of herself. Oporo fled the mainland and sought out Rapithwin for protection.

Meanwhile, Saturna made Ioana as a companion. She made other vampires to use as guards and eventually weapons against the humans. Rapithwin didn't make anymore vamps on the island, but she had been using them to collect souls for her ultimate weapon (which no one knows is buried at the bottom of the lake on Insula de Virsta).

Ioana intended to kill Edgar because of how heartbroken Saturna had been over his "betrayal." Instead she became his companion and lover. Rapithwin and Edgar eventually reunited and Edgar left Ioana to be with his former lover. Rapithwin became pregnant with Owen and Daevas. That was the thing that pushed Ioana over the edge to finally kill Edgar. Ioana blamed his death on the witches to hide the truth, which Rapithwin knew but could not justify without revealing Ioana's motives (her pregnancy, which Rapithwin had carefully concealed).

Upon the birth of her children, Rapithwin asked the only witch she trusted to age them. She put them into a sleep state until she was ready to hide them. She was clever about it, hiding them in plain sight though she led others to believe that she'd hidden them on the mainland. She introduced them a few years later as major players on the island. Rapithwin realized she couldn't raise her children alone (even as they were aged to adults). She went to the mainland one last time and begged Victor to return with her (as he had before politely refused). He consented and when they returned they both pretended that Owen had come from the mainland with Victor, as the final group of magicals to come to the island. Daevas had been tucked in with the Vatnadis as a limnatide, which had been missing since Rapithwin had killed the original. She pretended as though he'd always been in the lake and shifted everyone's memories to agree with this scenario.

Saturna is at the moment, at large. Rapithwin and Oporo have been doing their best to keep the island hidden from her and to maintain her banishment, but as Saturna has the same abilities they do it is only a number's game and a matter of time before one of them slips.