Since the Alicorn Amulet incident, Trixie suffers of nightmares. After some time she finally yielded and decided to get to the one pony who can help her. And so she returns to Ponyville, the place her nightmares began, not knowing that she has bigger problems than simple nightmares.
The dark tag is for two or three chapters that handle nightmares or past events with mentions of death. Those chapters will be marked with [D]