Twilight Sparkle has reached a plateau in her magical studies. In order to reach the next level of magical understanding, she conducts a dangerous ritual to summon a familiar, a knowledgeable creature from another realm.
She never expected him to be such a massive asshole.
Sex tag is for a passing reference. Nothing explicit.
What Celestia did sounds like a plot straight out of Star Trek: TNG. Domesticating your population using selective miscarriages is something Picard or Janeway would have many issues with and feel obligated to fix.