Moonfall was conceived as Diana's ultimate: it had a channel time, dealt true damage, and was powered by a resource called 'Scorn' (was replaced by Lunar Rush for being deemed toxic and almost uncounterable)
Diana is the daughter of foreigners who came to and died on Mount Targon, found and raised by the Solari. Diana always questioned the Solari's faith and was always drawn to Runeterra's moons.
After being indirectly drawn to Mount Targon's summit and chosen as the Targonian Aspect of the Moon, she shared her discovery of the forgotten shared history between the Solari and the Lunari with her elders, who branded her a heretic and sentenced her to death.
Leona was first drawn to finding Diana out of desire for avenging the elders killed by the Aspect of the Moon. After being chosen as the Targonian Aspect of the Sun, Leona is now seeking her out of a desire for answers about their shared knowledge of the Void.
Nami traveled to Ionia to find Diana and request she conjure a Moonstone to save the Marai from being overrun by the Voidborn crossing into the Guardian's Sea through a dimensional rift.