In the old lore, Emilia LeBlanc is not an individual but a mantle, with the current one (Evaine) being in charge of the underground society 'The Black Rose', whose members include Elise, Swain, and Vladimir.
Currently, it's still unknown if the "ageless" Pale Woman, aka Deceiver, had magically extended her life (as Singed had, albeit scientifically), or a long line of enchantresses successively adopted that alias, while keeping lower-ranking members in the dark.
LeBlanc set the stage for Swain's ascension to Grand General of Noxus.
LeBlanc had spearheaded Sion's initial resurrection (assumedly by dispatching Elise to the Shadow Isles, where she first met Vilemaw) and the current one managed to restore his mind with Jarvan IV's blood (the hemomantic ritual was conducted by Vladimir)
How she obtained the Exemplar's blood is unknown, but it is suspected his capture and imprisonment was led by Swain and then she herself impersonated the prince to avoid suspicion from Demacia.
LeBlanc is being targeted by Kalista (whom she betrayed specifically is unknown)