LUNA was founded shortly after the return of the Lunar Princess. Their main purpose was to remove any threats to Princess Luna's throne, at first. They quickly became feared through all of Equestria to all except the Royal Guards, the Princesses, the Princes, and the Element Bearers. Initially, LUNA only accepted previous members of the Lunar Guard, although they quickly made changes to allow Pegasi in as well. Now, they are contemplating allowing all races to join their ranks. The four races would still be divided by Regiment, of course. LUNA now only deals with possible opposition to the throne of Luna if the opposition is proven. Although, if the possible opposition gets wind of having LUNA on their tail, they stop all actions against the throne. The way LUNA deals with these would-be oppressors is not very pleasant.
Artwork by HotHead