What is The Moonstone in warriors?

Mothflight of WindClan was the cat do discover the Moonstone. She was a regular warrior that kept on drifting off looking for berries and leaves and wondreing how they could help cats. She was exiled from WindClan and walked until she reached the Moonstone. The warriors of StarClan told her that she would be the first medicine cat, and told her which cats to bring there every half moon, as the other medicine cats of the Clans. The whole story is in the Warriors Field Guide:Secrets of the Clans