Heroes of Hillsfar
There are pirates all over the Moonsea. Most are individual ships, or a group of two at the most. The only exception is the Black Moon Brotherhood, who were supported by House Marstel nobility in Hulburg (a town in the north east part of the Moonsea).
The Black Brotherhood were successful and powerful, and no one could find where they were based. They preyed on the ruins of Zhentil Keep, sacked towns and villages where they could, and destroyed other shipping and ransomed captives or sold them into slavery. Unknown to everyone the Black Brotherhood had magical ships that could actually fly to their base in Selune’s Tears (the asteroids orbiting Selune the moon). However they were eventually undone in 1479 DR by an opposing noble of Hulburg, Geran Hulmaster, and his adventuring companions, who disguised themselves as a pirates and stowed away on one of the magical flying ships, and caused a lot of damage to the base and to the ships with fire.
The Black Moon Brotherhood is a remnant of what it once was, and they are believed to no longer have their magical flying ships. Instead they are using the northern part of the ruins of Zhentil Keep as a base. They no longer have support from House Marstel of Hulburg, and are relying on what they can catch.