Exandria, created by Matt Mercer, is a setting for Dungeons and Dragons 5th Edition. Originally a homebrew setting for Critical Role, it became an official published setting by WOTC on 17 March, 2020, under the title of Explorer's Guide to Wildemount. It was first touched upon with the quasi-official splatbook Tal'dorei Campaign Setting, which takes place on a different continent in the same world.
The Tal'dorei Campaign Setting, published by Green Ronin Publishing, is an all-lore book that provides a history, geographical guide and racial writeup for Tal'dorei. While it had mechanical content (A subclass for Barbarian, Sorcerer, and Monk, as well as several other systems), none of it was official due to it being published by a third party and its content somewhat unpolished.
The Explorer's Guide to Wildemount is basically the same as the above, though focused on the continent of Wildemount, but also includes a lot of crunch, in the form of three new subclasses (the Echo Knight Fighter and the Chronurgist and Graviturgist Wizard), new monsters, new spells, new magic items, a new magic item subsystem based on the Weapons of Legacy, a new Elf subrace, a new Halfling subrace, two variant Dragonborn races, and reprints of the Sea Elf subrace from Mordenkainen's Tome of Foes, the Orc from Eberron: Rising from the Last War, the Aarakocra and Genasi from the Elemental Evil Player's Guide, the Tortle (the first time it's ever been in an official book!), and the Aasimar, Firbolg, Goblinoids, Goliath, Kenku and Tabaxi from Volo's Guide to Monsters.
Mercer has published other content related to Exandria such as a 'Blood Hunter' class and a Gunslinger subclass for Fighter, but none of that was official either.
Exandria houses all of the Player's Handbook Races, although Sea Elves exist here, as do the unique subraces of Pallid Elf and Lotusden Halfling. Dragonborn are divided into tailed Draconbloods and tailless Ravenite subraces. Additionally, it's also home to the three iconic Planetouched races (Aasimar, Genasi, Tiefling), Aarakocra, Firbolgs, Goliaths, Kenku, Tabaxi and Tortles, as well as non-evil Orcs, Goblins, Hobgoblins and Bugbears. It's also home to a unique "race" called the Hollow Ones, which are basically revenants of any of the aforementioned races.
One weird thing about Exandria is that, if you read the lore, there's a lot of interspecies shagging going on. Elf/Dwarf couples are a prominent thing in the Diarchy of Uthodurn (although the kids are randomly either dwarves or elves, with some minor aesthetic traits from their other parent), whilst the Half-Elf lore notes that the statblock covers things like elf-minotaur hybrids and elf-bugbear hybrids as well as the traditional elf-human hybrid. Orcs and goblins, however, refuse to interbreed, for fear of what might be unleashed by creating a child bearing the divine curses of both Gruumsh and Bane.
Again, all of the standard Player's Handbook Classes are here, along with three new subclasses based around the magical art of Dunamancy; a field of magic based on manipulating the primal energy of potentiality and actuality, which is known as "Dunamis". These classes are the Echo Knight subclass for Fighter (manifest an 'echo' of yourself to fight alongside) and the Chronurgist and Graviturgist wizards, who come with time and space themed spells, respectively.
There are also the aforementioned unofficial class of Blood Hunter, the subclass Gunslinger, and several subclasses from the Tal'dorei guide including the Blood Domain Cleric, the Juggernaut Barbarian, the Runechild Sorcerer, and the Way of the Cobalt Soul Monk (who can spend ki points when they attack to extract information about foe's stats and weaknesses). Oddly (perhaps due to publishing rights) the Cobalt Soul Monk wasn't printed in the Wildemount guide, despite one of the players in Mercer's Wildemount campaign taking that subclass.
Lore and Pantheon
While Exandria does have its own flair, it doesn't massively set itself apart by carving a niche the way settings like Dragonlance or Eberron do, and in many ways it's just a typical kitchen sink setting. It mostly acts as a 'fresher' alternative to high fantasy settings like the Forgotten Realms. To the chagrin of some, it is also noteworthy for its progressive undertones (there are a few LGBT NPCs mentioned in the Explorer's Guide and the setting tends to shy away from generalizing races as evil).
Exandria's pantheon is the Dawn War pantheon from the Nentir Vale, with the notable addition of Sarenrae from Golarion. In the Explorer's Guide, due to obvious legal issues, Sarenrae has been replaced with an expy called Raei. This is because Exandria was originally a homebrew setting for Pathfinder but was ported to D&D 5e when Critical Role became a show, and one of Mercer's players (Ashley Johnson) played a cleric of Sarenrae. Many gods are also often referred to by secondary epithets unique to the Exandria setting (i.e. Corellon is called the Archeart), which were seemingly established for the Tal'Dorei Campaign Setting book to avoid legal issues before Exandria became an official setting.
The good gods are called the Prime Deities, while the evil ones are called the Betrayer Gods, and they are sealed away from the Prime Material Plane by the Divine Gate. A number of quasi-deities also exist such as an archfey trickster-demigod called The Traveler (no, not that one) and a leviathan called Uk'otoa that was once Zehir's pet. The setting utilizes the Great Wheel cosmology with basically every major plane getting some sort of mention or relevance, while featuring some scientific concepts such as quantum theory and parallel universes via the discipline of dunamancy.
There are three major periods in Exandria's history: the Founding and the Primordial War, the Age of Arcanum and the Calamity, and the Post-Divergence era. The history of Exandria is also quite similar to the Dawn War lore - the Founding involves the gods creating the world, the mortal races, and the Metallic Dragons before coming into conflict with the Primordials, though in this case the Primordials are ancient elemental titans with little in the way of detailed lore. This age ends in the defeat of the Primordials and banishing of the Betrayer Gods. The Age of Arcanum includes the rise of mages and their quest for immortality; characters like Vecna and the Raven Queen first rose to power during this time. Eventually, the evil Betrayer Gods were re-summoned into the world, with basically all the forces of evil gathering in Xhorhas (essentially Mordor) and the Betrayer Gods launching an attack on the holy city of Vasselheim. This culminated in the Calamity, an epic battle between good and evil that killed two-thirds of the world's population, completely rearranged the geography and flow of magic in Exandria, and caused untold suffering. At the end of the Calamity the Prime Deities created the Divine Gate and sealed themselves and the Betrayer Gods behind it, an event known as the Divergence.
835 years have passed since then; the gods can only intervene in a limited capacity and the aftermath of the Calamity has resulted in essentially no shortage of monsters, ancient artifacts, or adventure hooks to find. The Vestiges of Divergence are legendary weapons that were used during the Calamity and are similar to 3.5e's Weapons of Legacy; they begin in a dormant state, but can be powered up to their Awakened and then Exalted state via pivotal moments of character growth. There's also an 'evil' equivalent, the Arms of the Betrayers, who are evil sentient weapons that require unique and difficult methods to destroy.
Exandria was popularized through a live-play series. As such, the adventuring parties from the show (Vox Machina and the Mighty Nein) are treated as canonical, although they are very rarely mentioned in the source books outside of passing mentions and easter eggs.
Exandria is a planet with two moons and a 328-day year of eleven months. It contains five continents. Oddly, the world's scale seems to be much smaller than Earth's; Wildemount appears to be the largest continent at 2200 miles across, making it just a bit smaller than Australia. In the first print of its campaign setting, Tal'Dorei's surface area was roughly that of Texas before it was corrected; it is now roughly 1500 miles long and 1000 miles wide, slightly larger than Greenland.
The level of technology is mostly medieval, with the exception of some steampunk societies like the gnome city of Hupperdook. Firearms were also recently invented, but have not seen widespread use.
Tal'Dorei (or Gwessar, as the elves call it) is a straightforward high fantasy themed continent with no clear real-world basis, and is governed largely by the Republic (formerly kingdom) of Tal'Dorei, though various other societies exist - the dwarf city of Kraghammer (which sits atop an entrance to the Underdark), the forested elf realm of Syngorn (which can shift into the Feywild in times of crisis), the druidic tribes of the Ashari (who guard portals to the Elemental Planes), the city-state of Whitestone (an aristocratic and technologically developed society), the militaristic hobgoblin federation of the Iron Authority, and even invisible flying cities of cloud giants. There are also organizations such as the Clasp (your token organized crime) and the Golden Grin (a very decentralized, Harpers-type secret society with an emphasis on freedom of expression and making people smile). Geographically it's mostly frozen mountains in the north, your standard plains and forests in the middle, and jungles in the south. The Campaign Setting takes place just as Tal'dorei is recovering from the ravages of the Chroma Conclave, an unlikely alliance of ancient chromatic dragons that attempted to conquer the continent before Vox Machina defeated them.
Wildemount is more fleshed out than Tal'Dorei, having the benefit of an official book, and features a greater emphasis on political intrigue. It has a more clearly European flair - the Dwendalian Empire being Germanic/Eastern European and the Menagerie Coast being more Spanish/Mediterranean culturally. The Explorer's Guide setting takes place in 835 PD (Post-Divergence, 20 years after the Tal'Dorei setting) and revolves around a war between two main factions:
The Dwendalian Empire is an authoritarian expansionist empire ruled by a paranoid king and enforced by a council of mages called the Cerberus Assembly, which in turn control the 'Volstrucker' or secret police who were more often than not kidnapped and/or brainwashed as children. The Empire has outlawed the worship of all but six gods and harshly levies taxes against the citizenry, but has a high approval rating due to their image as peacekeepers and protectors of the commonfolk.
The Kryn Dynasty (not to be confused with Krynn) is an all-inclusive society of monsterfolk led by drow who rejected Lolth and built cities of perpetual twilight on the surface; they worship a being of light called the Luxon and practice reincarnation with the help of artifacts called Beacons that are said to be fragments of the Luxon's body. The Kryn basically live in Exandria's Mordor, a ruined wasteland known as Xhorhas where many battles of the Calamity once took place (with the capital, Rosohna, built on the ruins of the evil city of Ghor Dranas).
The two nations are at war over the Luxon beacons, since the Cerberus Assembly have taken two of them in order to study them for their magical properties of Dunamancy. The Kryn are portrayed arguably in a more sympathetic light but are just as prone to propaganda and war crimes as the Empire.
Many other factions exist such as the neutral city-states of the Clovis Concord along the Menagerie Coast, the pirate nation of the Revelry, the librarian-monks of the Cobalt Soul, the crime syndicate of the Myriad, the tribes of Shadycreek Run, the elf-dwarf diarchy of Uthodurn, the blood hunters of the Claret Orders, and the dragonborn society of the Scars of Scale and Tooth (the remnants of the destroyed nation of Draconia, a caste system where tailed 'draconblood' dragonborn enslaved tailless 'ravenite' dragonborn). There's even a cult of Lolth known as the Children of Malice, who see the Kryn Dynasty as traitors and seek to destroy it by any means necessary. Wildemount is by far the largest continent in Exandria and contains many different flavors; for example, the Swavain Islands are suitable for your typical pirate adventures or hidden temples full of yuan-ti, or there's the northern islands of Eiselcross which contain the crash site of a former floating city of mages (Aeor) that is now permanently magically frozen in time and populated by their monstrous creations.
The Explorer's Guide to Wildemount features a character creation guide called the Heroic Chronicle and four mini-adventures for low level players.
The oldest continent, Issylra contains the region of Othanzia which itself contains Vasselheim, the holy city that has survived since the early days of Exandria. It, along with Marquet, has never been mapped in detail. It is supposedly the equivalent of Asia and Russia though Vasselheim is very clearly Nordic-based.
This is your Arabian Nights continent minus the unsavory elements, most notably containing the city of Ank'harel which is ruled over by an 'immortal' named J'mon Sa Ord who is actually an ancient brass dragon in disguise.
The Shattered Teeth
Probably the most mysterious continent, its true shape is a vague guess at best and the players on Critical Role have never traveled to it. It's actually an archipelago that serves as an analogue for East and South Asia, and is hidden behind a fog bank called the Fool's Curtain. It is dominated by two nations: The Ossended Host is based on isolationist Japan and its society is based around the worship of dreams and nightmares, while the Wanderman Assembly is a capitalist society based on the British East India Trading Company.