From publisher blurb:
Amarune's Almanac Volume 2 is the second book in a multipart series exploring the eight biomes of Dungeons & Dragons within the Forgotten Realms campaign setting.
The Underdark gives an overview of Faerûn's "Realm Below"; and its flora, fauna, and some history. It does this through both narrative prose, written from the perspective of Amarune Whitewave (the great, great granddaughter of Elminster Aumar), and through occasional notes from her husband, editor, and traveling companion Arclath Delcastle.
Many of the details in this book were vetted and seeded by the creator of the Forgotten Realms himself, Ed Greenwood, who is acting as a consultant on the project!
Contained with this 67-page tome, you'll get:
2 new subclasses
the Circle of the Dark druid which seeks to expand on the land (underdark) druid by channeling Faerzress.
and the Sharnbound, a conclave of Rangers who are tasked by the Sharn, and granted a limited form of their transformation, to act as agents of their will.
Variant rules: expansions to the Druid and Ranger classes to allow them more access to the new spells contained within this book!
10 spells: each using a new spellcasting component, Environment, as they each rely on the boons of the world around them.
+1 bonus spell, druidic practice, which is the Druid version of the ceremony spell for Clerics.
11 magic items: imbued with the power of the Underdark and its depths.
20 monsters: new creatures for Wild Shaping, mounts, and companions! Including the Phaerimm, Sharn, and Deep Dragons!
20 flora: the bounty of the Underdark. Mushrooms, lichen, moss, and even a few trees that you can collect and use to create all manner of items, as well as improve some spells.
Downtime activities: A gathering expedition specifically designed to find and collect the flora described in this book.
5 location stories: Straight from the journal of Amarune Whitewave herself, stories of their expeditions into five notable Underdark locations: Araumycos, the Burgeoning Rift, the Firelands, Glimmersea, and the Sharnwall.
A map of Faerûn: Updated for 5th Edition, this book contains a two-page map of the Underdark covering the Northdark all the way to Deep Raurin. As with any map of the evershifting Underdark, it's more of a guideline of where things are roughly located, than a navigatable map.