Secrets of the Astral Sea
From the Publisher's Website:
Climb aboard an astral skiff and set sail for adventure!
On the Astral Sea, characters face off against angels, devils, githyanki, and even the gods and their exarchs while exploring dungeons, raiding astral galleons, and establishing themselves as demigods. This game supplement builds on the overview of the Astral Sea presented in the Manual of the Planes game supplement and explores the heavenly plane in greater detail. From the cavernous layers of Nine Hells to the dark dungeons of Tytherion, adventure awaits in every astral dominion and on the vast, uncharted sea that stretches between them.
This game supplement describes the Astral Sea and key locations throughout the plane. It also presents a multitude of new monsters, as well as adventure hooks, encounters, hazards, and everything Dungeon Masters need to make the Astral Sea a featured setting in their campaigns.