Luna and Queen Chrysalis harshly rule Equestria.

How did this happen? Corruption, about 200 years ago. She hypnotized the guards magically and banished Celestia to the Everfree Forest. Fifty years ago she deemed earth ponies "unworthy" and began killing them off. In response a group in Ponyville turns earth ponies into pegasi and unicorns. Twilight is Luna's student.