This story is a sequel to The Cassandra Chronicles
This is Cassandra, my OC don't steal. Her interests include having a dark past, saving Twilight when the elements of harmony fail, and being Celestia's long-lost sister. When Equestria needs a protector, only the brave and beautiful Cassandra will rise to the challenge, as she struggles with the delicate balance between keeping the world safe and dealing with other ponies being jealous of her. Though I can't tag this as both Slice of Life and Adventure, the stories incorporate both elements. These stories will touch you, spiritually and emotionally; they will make you laugh, make you cry, and hopefully make you love Cassandra as much as I do.
Also check out the spinoff series "The Cassandra Crossovers" here: http://www.fimfiction.net/story/140779/the-cassandra-crossovers
Praise for The Cassandra Chronicles:
"wonderful. perfect and wonderful." -RagamuffinQ
"Too majestic for words." -Brony Hectic
"This is by far the best fiction on this site." -AwkwardTaco
"You show 'em, Cassandra!" -CartsBeforeHorses
"I love Applejack and this is still the greatest story ever told." -Twinkletail
"Your OC really is the best OC! (Other than MINE, of course...." -Twilicorn_Luver
"This is better then Fallout: Equestria." -DashieTheAlmighty