It is narrated by none other than Ulraunt, the Keeper of Tomes at Candlekeep, and also supports epic level play (as introduced in Epic Characters).
Here is a summary of what you will find:
Details on Acheron and each of its four layers, including environments, fauna, and flora, how to get there, what hazards to avoid, and much more.
A detailed look at each cube face of many of the cubes that exist on the various layers, such as Nishrek, home of the orc pantheon, and Clangor, home of the goblinoid pantheon.
Tables to help you quickly create your own cubes.
We explore a total of 16 deities, including Bane, Laduguer, Deep Duerra, Urdlen, and the entire goblinoid and orc pantheons.
Each deity includes nifty tips for roleplaying them, mighty stat blocks, awe-inspiring lairs, and templates for creating versatile avatars.
One new playable race and one new subrace.
Eighteen new and unique subclasses, such as the exploration-focused surveyor artificer, the sinister blood hunter of the order of the warmonger, or the malleable druid of the circle of warshaping.
Over 50 new spells of all levels, each inspired by the prevalent themes of Acheron such as atrophy, cold, magnetism, metals, psionics, and storm.
138 new stat blocks, some based on existing lore, others seen here for the first time.
Four war mounts.
Three new war machines.
Seven unique characters from the Forgotten Realms, such as the fabled Obould Many-Arrows.
- from the publisher's blurb