All you see here is connected to Prequel world that is meant to tell the stories of Princesses, from the very beginning of their lives till all the events know to you from the MLP FIM show and even after that.
I know English is not your first language. Neither is it mine, but I like to think I've become quite adept at it. I just wanna point out some spelling mistakes in the text here, if you don't mind:
1. I think you need to put an apostrophe "s" at the of the word "Moon" so that it reads: Nightmare Moon's return...
2. I think the last part of the second phrase should read: ...Princesses, WHO once more locked villains in the mighty depths of Tartarus Gates. Please note that there is no comma between the words "depths" and "of". There should not be one there to begin with.
3. Third paragraph: I think the second phrase should be worded like this: ... someone they would never have expected to see as their enemy: Discord, THE Lord of Creation; WHO decided... The rest of the phrase looks OK to me. Please note there should not be a comma between the words "someone" and "they". There also should be a "the" before the word "Lord"; and "who" should also be before the word "decided".
4. Third paragraph, last phrase: It should read: Yet even his powers WEREN'T enough... Because you're using "powers" in the plural sense.
5. Last phrase: It should read: ...another foe, one that is willing to control the entire world... A comma instead of a period after the word "foe" makes this a complete sentence. Also, no double "l" in "control"; and you forgot the "the".
Please don't take this the wrong way. This is just a little constructive criticism.
Wait wait wait. The princesses defeated Tirek? As in THE Tirek? As in the Rainbow of Darkness, Night that never ends, Midnight Castle, holy s@#$ he's going to kill everypony and everyone you ever love Tirek?
Also, wasn't Grogar merely banished to Tambelon not Tartarus?