Dark Spire- the son of Starlight and Sunburst learn about his skills in magic and become arrogant, aggressive and darker. One time he learns about another pony who was like him, and the same pony named Sombra calls for him, teaching him and turning him to his apprentice.

Children of Grogar- the cult that worship grogar and want to destroy The main 6 and discord for using their master's magic and destroy his honor.

I like the first character idea, definitely.

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I based him on kylo ren.

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I think I can use him for a story about a new order of ponies with an army of young starlight clones.