In her new lore, this secretive, ageless Pale Woman betrayed her superior, the warlord-king Mordekaiser & former ruler of the Immortal Bastion, to the barbarian invaders who would later found Noxus, LeBlanc being only one among her many aliases.
She would later inexplicably manipulate nations to war against each other for centuries,for instances, she masterminded the Noxian invasion into Ionia.
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.
Leblanc had helped Boram Darkwill to Grand Generalship, only to 'betray' him & be executed by Darkwill's loyalist Swain.
After staging her 'death', she falsely promised the decrepit & mentally unstable Darkwill with immortality & convinced him to send Swain to his death or disgrace in the disastrous Ionian campaign.
Still, Swain survived & discovered the truth; he then violently usurped Darkwill's throne with a newfound power & subjugated the Black Rose, at least for the time being.
In her old lore, 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.