Short answers, pulled from the story.
Explorer's Guide to Wildemount is a 304-page sourcebook for the 5th edition of Dungeons and Dragons, published by Wizards of the Coast on the 17th of March 2020. It details the continent of Wildemount from Matthew Mercer's Critical Role campaign setting, including four distinct regions, new player options, and a new school of magic called dunamancy.
Explorer's Guide to Wildemount was released on the 17th of March 2020 by Wizards of the Coast. It was announced on the 13th of January 2020, one day after an unauthorized Amazon product listing had leaked its existence.
The world of Wildemount was created by Matthew Mercer as the setting for the Critical Role web series. Wizards of the Coast published the official sourcebook, with Chris Perkins serving as internal lead on the project.
Dunamancy is a school of magic introduced in Explorer's Guide to Wildemount that covers forces of time, gravity, and fate. It adds 15 new spells and two new wizard subclasses, Chronurgy Magic and Graviturgy Magic, along with the Echo Knight fighter subclass.
Through pre-orders alone, Explorer's Guide to Wildemount reached number one on Amazon's overall bestseller list, outpacing New York Times bestsellers. In its first week of release, Publishers Weekly ranked it first in Hardcover Nonfiction and fourth overall, with 26,589 copies sold.
The book adds four new player races: Pallid Elves, Lotusden Halflings, Draconblood Dragonborn, and Ravenite Dragonborn. It also introduces the Echo Knight fighter subclass, two new wizard subclasses, and the Heroic Chronicle character creation system, along with over 20 new monsters and the Vestiges of Divergence magic items.