The Story of Morgana
Morgana and Kayle are twin sisters — the daughters of the Aspect of Justice who was sent to Runeterra, and a mortal woman who died giving them life. Their father, the Aspect, raised them in his divine tradition: absolute justice, the kind that does not bend for mercy. Kayle embraced this. Morgana looked at how absolute justice operates in practice — who it falls on, what it costs people who are already broken — and refused it. The split between them is theological and is also entirely personal.
She fell. That is how Kayle understands it: Morgana's refusal to participate in their father's mission, her choice to remain on Runeterra and live among the mortals rather than ascend to divine judgment, was a fall from grace. Morgana experiences it differently — as a choice to stay, to be present to the suffering that divine detachment overlooks. She chains the unjust with dark magic and walks in shadow, and she does this as a rejection of the righteousness that would have her ignore human cost in favor of divine principle.
She is in Demacia now, working — quietly, carefully — to protect mages from the Mageseekers. She uses magic that looks dangerous because it is dangerous, on behalf of people who have no other protection. Kayle would call this corruption. Morgana would call it the thing justice actually requires when you're close enough to see who gets hurt. They haven't agreed in centuries. They are still sisters.