Vespera was locked in Lumenaria for an unknown amount of time for her human experimentation , until Fintan Pyren, Gethen, and Ruy Ignis broke her out (Brant was there, but was killed by fallen debris, and Oralie). She created the original Nightfall, which Lady Gisela based her version of Nightfall off of.
She also was the one who (according to Councillor Bronte after translating the journals Lady Gisela provided) caused the skirmish between elves and humans, not because humans were greedy, but because she was abducting hundreds of humans in Atlantis and the old Councillors did not pay any attention to the humans' reports, complaints, and pleas.
She is an Empath. Keefe speculated that she felt too much with her empath powers and went numb, not caring about anything anymore. This explains how she is able to torture humans when elves are supposed to go catatonic or have their mind broken.
In the end of Book 6: Nightfall, she joins forces with Lady Gisela because it was the best option over imprisonment, though she still thought that Lady Gisela's views were incorrect. Vespera sees her as a person who took her dream (Nightfall) and did not see the point of it and built a flawed version of it. When Sophie and her friends went to her Nightfall to rescue Sophie's parents, she showed herself in an illusion of mirrors and light. She and Biana fought against each other when Biana managed to sneak into the secret place where Fintan Pyren and Vespera were hiding, and she smashed her into the mirror. Biana got thin, permanent scars on her arms, neck, and back as a result.
In Flashback, it was Vespera's voice that caused Alvar to regain his memories.
(From the KotLC Wikia)