From publisher blurb:
A New Realm of Adventure
The Shadowplane setting is designed to be able to add onto any existing setting using the 5th Edition OGL. Though it has a unique history and presents unusual approaches to even the most familiar elements of 5e (as you will soon discover) the Shadowplane is meant to plug into any setting. If a setting already has a plane of shadows or darkness as part of its established canon, the Shadowplane can easily coexist as a demiplane or pocket dimension adjacent to the traditional and more well known plane of shadows.
The Twilight Exodus
In ancient times, shadow dragons lived among the chromatic and metallic dragons of the drachiarchy. While the other dragons squabbled over moral quandaries and the acquisition of treasures, the shadow dragons turned their attention to the realm of shadow.
In the course of their studies, the shadow dragons unlocked powerful magic. Proud of these achievements, the shadow dragons showed the drachiarchy their discoveries. The drachiarchy demanded the shadow dragons hand over all their research to the rest of dragonkind so that all might benefit from this new power. The shadow dragons refused, claiming dominion over the Shadowplane and its secrets.
The drachiarchy didn’t handle this rejection well. The chromatic and metallic dragons set aside their squabbling long enough to mount a joint offensive against the shadow dragons. This was a swift and brutal attack which cut the population of shadow dragons and their followers in half over the span of a fortnight. Many shadow dragons died rather than hand over their knowledge.
Those who survived the Torchlight Crusade — as it is now known — gathered their belongings and followers in a grand meeting at the eldest shadow dragon’s lair. Once all were present, the shadow dragons enacted a potent ritual that moved themselves, their followers, and the land immediately surrounding the lair into the Shadowplane. This event became known among the denizens of the Shadowplane as the Twilight Exodus.
Guide to the Shadowplane Contains:
3 new playable subraces
4 new subclasses
18 new spells
11 new magic items
10 new NPCs
8 new monsters
A starter adventure
And a young Shadow Dragon’s lair!