Another author who decided to add to her trilogy years after it finished. Crown of Shadows was published in 1995 and in 2012, C.S. Friedman wrote a novella prequel to her Coldfire Trilogy which I again listened to as an audiobook.
It gives some background to how Gerald Tarrant decided to make the forest his home and found Amoril. But it isn't necessary for the series. I was hoping for a story about the original settlement and the colonists.