From publisher blurb:
Explore the ruins of the fallen empire of wizards who invented the ninth-level spells with new sub-classes like the Portal Assassin for the Rogue, using teleportation to stab enemies and then quickly run away, or the Oozemaster Artificer, brewing up little blobby buddies to help you out in a pinch. And if you really like being ooze-y, you can take the ooze transformation, swapping out bones and organs for an undifferentiated mass of acidic goo. Finally, 5e has playable goo girls!
The book also has nine hit die powered backgrounds giving you something to do with all those hit dice you never spend on healing during short rests, like casting spells or permanently reducing your hit die maximum - but not your maximum HP - for an extra save proficiency. At this stage in the series, most of the new hit die powered backgrounds are usually reprints from old books, but not this time, as five of the nine have never been seen before in any previous book.
In the GM’s section of the book, we have several new stat blocks for magical remnants of the Mana Kings, like the transmutation horrors of the Shapeless Sovereign, the elemental servitor golems of the King Betwixt, or the warped sinspawn mutants of the Crimson King.
And the book will add the titular ruins of the Mana Kings to the Creative Commons Weskven setting. You can find a link in the transcript to the existing Creative Commons content from the first seven regions of the setting. The free version is a plain-format Google Doc with no illustration, but all the setting material in that document is released under a Creative Commons 4.0 International Attribution-Only License for use by any creator in their own work, including commercial work, and all they have to do is provide attribution to the source. They do not have to release that work under the Creative Commons, that is the Creative Commons Sharealike License, which is different from the Attribution-Only License that Weskven is released under.
With the release of this book, the Creative Commons will receive a description of the Mana Kings’ ancient magepunk empire which destroyed themselves by sucking the earth dry for magical power with huge mana towers, leaving behind ruins full of magical wonders to be picked over by the mostly-gnomish Glimmfall Scholars, who now seek to excavate the deadly remains of the Mana Kings for both archaeological and arcane research.