While in the human world, on a date with Sunset, Twilight discovers a badly injured child that she recognizes a little too well with a newborn baby. Taking them to a hospital, they discovers more about the children and decide to take them to Equestria to help them out.

Nyx is from Past Sins, read it here:http://www.fimfiction.net/story/41596/past-sins
Gore tag for chapter 4.