Darkness has attempted to run and even fight back against the light for an eternity, and one of its many tools is the ability to make a copy of a world (with restrictions however). Nigtmares are creatures that once were a part of the darkness, but betrayed the darkness to work for the light. Nightmares have long used the fears of mortals to track down darkness that plagues them, hence their name. Dusk is amongst the great in the nightmares and has been sent to purge a copy of the wolrd the ponies exist. Due to thier lack of a true physical form, nightmares require "vessels", or a living creature that will allow them to inhabit their body. Dusk and his vessel, Kindrell, come into this plagued world, and work to free it.

When your life is as dull a gray as the world that surrounds you, the mundanities can make it all seem meaningless. Sometimes all we need is a little color -- or six -- to reintroduce us to what truly makes life worth living.

*6-16-2017 Edit, added Rainbow Dash tag because it triggered knighty.*