So... I read the raws and I think this is her 3rd reincarnation?
In her 1st life, I think she's some sort of celestial being. She died (more like faded) after sacrificing herself to use the divine power to purify the undefeatable dark beast, to protect onsaemiro. The divine power is said to be usable by fairies, so I guess she achieved being one apparently???
In her 2nd life (the one we saw in the prologue), she's a daughter of a general king or something. Likely due to a malicious intent of using him for war (abuse power or requires his sacrifice), she stole the urn and plunge herself into the sea, thus drowned.
Will she still sacrifice herself this time?