You've finally joined the Equestrian Royal Guard! A little too late though, as you're now serving the Queen of Darkness. But, just because the environment has changed doesn't mean that you can't do the job you've always wanted to do.
And Nightmare Moon seems to have similar feelings about her own role.

Warning: contains lots of ponies killing and being killed. Not in inherently disturbing ways, but death is still a pretty big deal.