From a lore stand point, LeBlanc has been alive well before Noxus and is this Pale Lady that everyone knows the tale of. LeBlanc likely has been the cause of many wars throughout history, but the problem is the two things that we know that the deciever has done ultimately failed. LeBlanc banished Mordekaiser to the Shadow Realm where he now resides, thing is, that was his plan after all and now has an empire in the afterlife. Next we have The Invasion of Ionia, which LeBlanc was clearly the mastermind of. Having Darkwill as a puppet, Swain's troop was surrounded and Swain almost died.
Another really interesting thing about LeBlanc is her unique magic, she's older than Noxus by centuries therefore she's ancient but uet she still looks young. She has the power to appear at multiple places at the same time via clones, enabling her to fake her execution multiple times. But the real kinder is that she can grant her magic to others such as giving Elise everlasting youth, enchanting Riven's blade, and other things.