Hmm... I think I managed to decipher that correctly.
HUGE ETERNAL POISON SPOILERS, DO NOT READ!
Okay, so Thage, Queen of Valdia, wife of Belfour/King Valdus, was killed when Valdus got hungry for power (or fell in love with the Goddess of Majin, Izel, not entirely sure which it is). He sacrificed her to try to give Izel an earthly body. For some reason, it didn't work, and Thage was reborn in a new body. At some point, the Guardian deity of the Dark path of the 3rd level of Besek 'calls the Darkness' to Thage, and she decides she wants revenge for being sacrificed. To get revenge, she's going to destroy the world.
She decides she can do this with the Eternal Poison (the Poison Pink, apparently, in Japan), which is thought to be some kind of wish-granting artifact. It is, in fact, some kind of mystical bond that is formed between a summoner and Besek when they summon Besek to a plane of existence. The summoner's blood becomes the Poison Pink. This also keeps the summoner from being killed permanently, by causing a 'Time Echo' should the summoner die. Time resets to the moment that the summoner's killer entered Besek, and events in both Besek and the outside world can occur differently. King Valdus proves to us that the holder of the Poison Pink doesn't always remember the previous events that occurred. However, it seems that anyone who holds a Librum -does- recall the events, as evidenced by Thage and Morpheus.
I'm not sure how it helps Thage to destroy the world, but she kills King Valdus once, and watches on each time someone else is able to kill King Valdus, collecting the Poison Pinks that are created each time his blood is spilt, for a total of 5 Poison Pinks, along with the Librum Vespera. The final time that King Valdus is killed, Duphaston uses the now-completed Librum Aurora to counter the Poison Pink's 'Time Echo' effect and truly end Valdus. This destroys the Librum Aurora in the process, leaving Thage's Librum Vespera unchallenged in power. She then convinces everyone to destroy the Goddess Izel, who had been reborn in Thage's daughter's body due to Valdus' ritual.
Unsurprisingly, all of that would allow Thage to destroy the world unchallenged.
Things that don't fit in - Why does Thage care about Retica? What did Ranunculus hope to get Thage to do? Where did Thage's new body come from, especially considering that the Majin seemed to think she was Izel (like Ashley and Lenarshe), so one would assume that the new body has to have the trace of Majin blood in it too. Why does Ashley treat Thage like a childhood friend (if the relationship there is just that Ashley recognizes Thage as her mother... the timing doesn't really line up, Thage was sacrificed 14 years ago or more and Ashley was born 17 years ago... just feels like there's a missing piece there, especially given the way Ashley acts around Thage when they first meet).
END ETERNAL POISON SPOILERS
