Diminutive, mysterious & inviolate race who control the Fallships which ply the vertical waterways linking the realms of the Underdark.
The Boatmen take payment in peculiar coinage called Dash for their services, which comprise transport up & down the eternal waterfalls that link the different realms of the Underdark.
It is well-known in the Underdark that no-one crosses the Dark Boatmen nor causes trouble on their ships, not even the Aboleth, so travel on a Fallship is always perfectly safe.