This story is a sequel to The Magician, and Her Timid Assistant- a Genetic Research story

After Fluttershy and Trixie part ways at the Griffin Kingdom, Trixie decides to follow up on the worries she'd shared with the pegasus. A tip brings her to Detrot, a place she thought her father had never been. There, she may finally understand what drove Lulamoon the Magnificent's previous actions.

Part seven of the Genetic Research series. Start at the beginning here
Sequel here