Unearthed Arcana
Unearthed Arcana is terminology taken from Dungeons & Dragons and is essentially more or less what you get when the game developers take a shot at homebrew.
In Advanced Dungeons & Dragons, Unearthed Arcana was a rules supplement written by Gary Gygax and published by TSR in 1985. It was, essentially, a cash grab; TSR was pretty heavily in debt, so Gygax's brilliant idea was to gather up a bunch of D&D content of various types published in Dragon Magazine (mostly by himself) or which had been submitted there but never printed, stick them all in one easy-reference book, and flog it to the market as a third "core" book, alongside the Player's Handbook and the Dungeon Master's Guide.
The result was... less than brilliant. To put it in perspective, in that year's November issue of Dragon, there were four pages worth of rules corrections, a two-page list of typo corrections, and some explanations and justifications for items which were not actually errors. In January 1987, that month's issue of Dragon devoted the entirety of its "Sage Advice" column to answering readers' questions about Unearthed Arcana, as a follow-up. Even then, it took until the 2013 anniversary reprinting before an edition of the AD&D Unearthed Arcana was published that actually incorporated this stuff.
Content included in the AD&D version of UA included the first ever version of the Barbarian class, alongside the cavalier and the thief-acrobat, new races/subraces like the drow and the svirfneblin, new weapons, revised information on character level maximums for non-human player characters, revised weapon specialization rules, new spells, the Comeliness ability score, magic items and non-human deities, alongside others.
Wizards of the Coast would later use the name twice over.
The first time was for Dungeons & Dragons 3rd Edition, where a second rules supplement was posted in 2004. This version of the UA was akin to its predecessor, in that it was marketed as a new tool for dungeon masters and veteran players. However, rather than just containing new content - which was, after all, the norm for every supplement in 3e by that point - this version of Unearthed Arcana focused on various ways to customize existing rules. Where the original Unearthed Arcana had simply expanded the rules and options of the core game, this 224-page supplement was aimed at providing an extensive list of variant rules and options to change the standard game itself. The book ends with a checklist of the included variants, preceded by a short chapter discussing ways of transitioning among multiple games using different rulesets (one of which explicitly emulates the "Eternal Champion" stories of Michael Moorcock).
The second use of the name by WoTC was tied to Dungeons & Dragons 5th Edition. In February 2015, WoTC began publishing a semi-regular series of PDF articles on their website, displaying homebrewed, experimental content created by WoTC's writers and which could potentially be updated into official content in the future, depending on fan-reception. Usually, only one such article gets published a month, but sometimes WoTC gets motivated and releases more than one per month.
List of 5e UA Articles
2015
February
Eberron Update: Bare-bones mechanics for converting Eberron campaigns to 5e mechanics. Includes 5e PC stats for the Changeling, Shifter and Warforged, Dragonmark rules, and the first rendition of the Artificer, as a Wizard subclass. More or less panned by fans of the setting, who feel most of it was phoned in - the Artificer was particularly loathed for being underpowerd, whilst the Warforged was so weak that Keith Baker himself put up his own version on his blog.
March
When Armies Clash: Rules for mass combat in 5th edition.
April
Modifying Class: As the name suggests, an extended discussion on how to mechanically tweak and homebrew classes and subclasses. Examples give are a non-spellcasting version of the Ranger class and the first edition of the Favored Soul as a Sorcerer class.
May
Waterborne Adventures: Rules for playing naval campaigns, complete with the Krynnish Minotaur race, the Mariner fighting style, and the first drafts of the Swashbuckler and Storm subclasses for the Rogue and the Sorcerer respectively.
June
Variant Rules: The first of several UAs aimed at modifying and expanding the rules of 5e as a whole. This article contains the "Players Make All Rolls" rules variant, the "Custom Alignment" rules variant, and the Vitality optional rule.
July
Awakened Mystic: The first rendition of the Mystic class, 5e's first attempt at bringing psionics back into the game.
August
Modern Magic: 5e rules aimed at more "Urban Fantasy" games, ala D20 Modern. This particular article contains the City Domain for Clerics, the Ghost In The Machine Patron for Warlocks, and the Technomancer Tradition for Wizards, as well as new spells for all of them, and a gunslinging invocation for Warlocks. It ties into another article called "Behind The Screens: My New D20 Modern Campaign", which also features more urban fantasy equipment in the form of new firearms (sidearms and longarms) and modernistic armors.
Technomancersrepresent a Wizard for a more modern age, one who focuses on mastering the interplay between magic and technology. A fairly fluff-accurate character, but unlikely to be of much use unless your DM wants to run either an urban fantasy game or a magitek/dungeonpunk game in the vein of Shadowrun - or at least with more advanced stuff than Eberron. The Technomancer gets free proficiency with sidearms (that is, pistols) and hacking tools at level 2, as well as trading their spellbook for an enchanted tablet computer. This lets them scribe spells in half the time of a normal wizard. At level 6, they get Program Spell, which lets them use any basic computational device (mobile phone or better) to "hold" a spell for them. The charge only lasts 48 hours, is one-use only, and a technomancer can only have one at a time. On the other hand, this lets you give somebody an undetectable improvised explosive device by handing them a cheap mobile primed to detonate a Fireball when they answer it. At level 10, they get Online Casting, where they can use networked electronic devices to channel spells for them. So, if you can get yourself into the mall's security room, you can promptly blast every fucker who walks into a room with a working security camera, without leaving the place. Finally, at level 14, they get the Chained Device feature, where they can have a tablet computer or better gizmo maintain concentration on a spell for them, allowing them to hold two concentration spells at once.
September
Ranger: This month's edition presents a 1st through 5th level look of an updated, revised ranger, born as a result of all the people complaining about just how awful the default 5e ranger was.
October
Prestige Classes & Rune Magic: As its name suggests, this article tries to bring back the concept of prestige classes, and introduce the Rune Magic subsystem on the side. The response was lackluster and PrCs don't seem likely to ever officially come back, though that might be due to how bad the Rune Scribe sample PrC was.
November
Light, Dark, Underdark!: As you can probably tell, this UA was inspired by the Underdark. Two fighting styles - the Close Quarters Shooter ad the Tunnel Fighter, the Deep Stalker Archetype for Rangers, the Shadow origin for Sorcerers, and the Undying Light Patron for Warlocks.
December
That Old Black Magic: Demon-focused UA article, presenting the new Abyssal Tiefling variant race and new Wizard spells for conjuring various low-level fiends.
2016
January
Kits of Old: New subclasses based on various kits from Advanced Dungeons & Dragons, specifically for the Bard (College of Swords, College of Satire) and Fighter (Cavalier, Scout). None were seen as particularly bad, although Cavaliers did elicit grumbles about how they fall into the same old traps and Scouts elicited outrage from Ranger fans for being better at their job than Rangers are.
February
Psionics And The Mystic, Take Two: An update and expansion of the original Mystic from the previous year, now going all the way up to 10th level. This one doesn't get a lot of attention because WoTC has screwed it up putting it into their own archives, forcing anons to search for it with the clumsy search engine the site boasts.
March
March 2016 Review: One of the lazier Unearthed Arcanas, this article consists of WoTC shilling the three DM's Guild products they most took a liking to, stating that if fans agreed with this like, such material could be ultimately made official content. The three products advertised were Blood Magic, Book of Beasts: Demon Depository, and Battle for the Undercity. Fans were... not impressed.
April
Gothic Heroes: Introduced the Revenant PC race, the Monster Slayer subclass for Fighters, and the Inquisitive subclass for Rogues.
June
Feats: Just what it says on the tin; a bunch of new experimental feats, these ones focusing on weapon and tool proficiencies, like the official Polearm Master feat. Fell Handed, Blade Mastery, Flail Mastery, Spear Mastery, Alchemist, Burglar, Gourmand, and Master of Disguise all appear in this article.
July
Quick Characters: Optional rules intended to make generating characters quicker and easier to pull off.
August
The Faithful: New subclasses to allow arcane casters to get in on the divine casters' schtick. This article introduced the Seeker Patron for Warlocks, and the Theurgy tradition for Wizards. There was an immediate uproar that the Seeker was all over the place and the Theurge was overpowered.
Theurges are incredibly powerful, and this has led to quite a bit of backlash from fans who denounce them as being the most broken Arcane Tradition in the game, a title they held until the Lore Master came out in 2017. Essentially, a Theurge is a "wizard-priest", an arcanist who is either devoted to a deity sufficiently to wield some of their magical powers, or has learned to imitate (or usurp) divine magic through arcane lore. Essentially, the Theurgy Tradition is an alternative to the Arcana Domain for Clerics, in that it tries to make a singular class out of the Cleric/Wizard Mystic Theurge, but by from the basis of being a wizard who studies and wields godly magic. Amusingly, by its default fluff, you can easily use it to represent an Ur-Priest as well. It's first second-level feature is, of course, Divine Inspiration, which lets you pick a specific Cleric Domain. Naturally, this should be one appropriate to the deity your Theurgist is worshipping/studying/ripping power from, though it notes that the most natural fits for a Theurgist are probably Arcana, Knowledge and Light. Its second level 2 feature is Arcane Initiate; when you gain a level, you can choose to trade one of the Wizard spells you know for a Cleric spell from a slot level you can cast - so, if you can cast 3rd level spells, you can snag a 1st, 2nd or 3rd level Cleric spell. You must concentrate on gaining the bonus spells from your chosen Domain first; only if you can cast all of the spells from that Domain can you pick up "generic" Cleric spells for your spellbook. Additionally, though you keep these spells in your spellbook, other wizards can't learn to cast them by copying from your spellbook. Its final 2nd level feature is Channel Arcana, which basically gives you Channel Divinity 1/short rest (2/short rest at 6th level, 3/short rest at 18th). When you make use of this power, you can gain either the Divine Arcana power-up (+2 to attack roll or saving throw of the next spell you cast) or the Channel Divinity effect granted by your chosen domain. The rest of its Tradition features are based on gaining access to your Cleric Domain's bonus features, with the exception of weapon and armor proficiencies. Arcane Acolyte (level 6) gives you access to the level 1 bonus feature, Arcane Priest (10th level) gives you access to the level 6 bonus feature, and finally Arcane High Priest (level 14) gives you access to the level 17 cleric bonus feature for your domain.
Arcane High Priest is the most commonly cited and least legitimate source for complaints about the "overpowered" status of Theurges, with many falsely insisting that, RAW, you could take the Arcana Domain and thusly get access to Wish at level 14. More legitimate complaints focus on the potential for abuse if you apply Spell Mastery to Cleric spells,or the fact you can effectively have two Domains by becoming a multiclassed Theurgist/Cleric.
September
The Ranger, Revised: A follow-up to the UA from a year ago, this further expands the Revised Ranger's ruleset by making it a full 20th level class at last.
October
Encounter Building: New rules to make generating encounters quicker and easier.
November
In this month, WoTC introduced a glorious but ultimately short-lived changeover to make UAs come out more than once per month.
Barbarian Primal Paths: First edition of the Ancestral Guardian, Storm Herald and Zealot subclasses.
Bard Colleges: First editions of the College of Glamour and College of Whispers subclasses.
Divine Domains: First editions of the Forge, Grave and Protection subclasses for Clerics.
Druid Circles and Wild Shape: First editions of the Circles of Dreams, the Shepherd and Twilight, plus alternate rules for governing druid wildshaping.
December
Martial Archetypes: First edition of the Arcane Archer, Knight, Samurai and Sharpshooter subclasses for Fighters.
Monastic Traditions: First edition of the Way of the Kensei and the Way of Tranquility for Monks.
Sacred Oaths: New material for Blackguards, in the form of the first editions of the Oaths of Conquest and Treachery for the Paladin.
2017
January
Artificer: The second ever fully-fledged new class for 5e, in the form of a new take on the Artificer, complete with the Alchemist and Gunsmith subclasses.
Ranger and Rogue: First edition of the Horizon Walker and the Primeval Guardian for the Ranger, and first edition of the Rogue's version of the Scout.
February
Sorcerer: First editions of the Phoenix Soul, Stone and Sea Origins for Sorcerers, and the second edition of the Favored Soul Sorcerer subclass.
Warlock and Wizard: First editions of two new Patrons for the Warlock, in the Hexblade and the Raven Queen, four new Warlock invocations, and the first edition of the Lore Master Tradition for Wizards, which has been derided as being even more broken than the bloody Theurge was.
Lore Masters were... well, if you thought Theurgists got bitched about, you hadn't heard anything yet. Lore Masters are the return of the "Generalist Wizard" archetype, which is represented by giving them quite a bit of versatility, attracting complaints that now it outdoes the 5e Sorcerer at being the "versatile caster" as well as complaints about being overpowered. Its first 2nd level feature, Lore Master, doubles your proficiency bonus for Arcana, History, Nature and Religion checks (you still gotta be proficient in the first place) and lets you roll Initiative based on your choice of Int or Dex. The second level 2 feature, Spell Secrets, lets you change the damage types of elemental damage spells (that is, spells that inflict Acid, Cold, Fire, Force, Necrotic, Radiant, or Thunder damage) at will, and change the saving throw type of a single spell per encounter. This means that not only could you throw around all of the "force damage inflicting fireballs" that you want, you could also throw a fireball that is resisted with a Strength check once per encounter. Level 6's Alchemical Casting lets you modify certain spells by burning an extra spell slot; give up a 1st level spell slot when casting a spell that inflicts damage for +2d6 bonus Force damage, give up a 2nd level spell slot when casting a ranged spell of at least 30 feet to instead upgrade its range to 1 mile, and give up a 3rd level spell slot when casting a spell with a saving throw to increase its DC by +2. At level 10, you gain the Prodigious Memory trait, which lets you use a bonus action to swap one of your memorized spells out for a prepared spell instead once per encounter. Finally, level 14 makes you a Master of Magic, letting you cast 1 spell from any spell list (must be a level you can actually cast for, though) once per day. This tradition has received a huge outcry for effectively being better at magical flexibility than the entire Sorcerer class is, to say nothing of their ability to make all their damage Force damage, and therefore nigh-irresistible.
Mass Combat: A revision of a concept touched upon in the very second every UA Article, way back in March 2015.
Traps Revisted: New rules for DMs to handle managing, placing and disarming traps.
March
The Mystic Class: The complete version of the Mystic, so far, a full 20th level class with all the mechanics that 5e plans to use to handle psionics, or so it seems.
Wizard Revisited: A second edition revamp of the Theurgy Tradition and a new Tradition in the form of War Magic.
Warmages were released in a March 2017 Unearthed Arcana, and if you think about it, they make more sense in this edition, as Wizards and Sorcerers no longer use different casting styles, and they were always highly trained, which is the wizard's "thing". Surprisingly, they're not redundant nor do they step on the toes of Evokers; they're all about blasting smarter, not harder. The first of their second level features, Arcane Deflection, lets them boost up their defenses with magical energy - when they take a hit against AC or fail a Con saving throw, they can burn their reaction to immediately boost their AC by +2 or their Con saving throw score by +4, as needed, in order to negate that particular attack. The downside is that they can't cast anything more powerful than a cantrip until the end of their next turn after using this trait. Their other level 2 feature, Tactical Wit, lets them add their Int modifier to their initiative rolls. Power Surge, unlocked at level 6, allows them to boost up their crowd-smiting attack spells once per short rest, in the form of adding +2 damage dice to the spell - this means they can launch a 10d6 fireball once per encounter, and at hitting level 20, they could unleash a 16d6 fireball. Their 10th level feature, Durable Magic, continues the "mystical protection" theme, giving them +2 to AC and to all saving throws whilst they have a spell maintained. Finally, the level 14 Deflecting Shroud provides a nice mixture of defense and offense, as it lets the Warmage deal half their level in Force damage to all enemies within 10 feet whenever they use Arcane Deflection. This Tradition was made official in Xanathar's Guide to Everything, with Power Surge getting a complete rewrite, Arcane Deflection boosting all saves, and Deflecting Shroud gained greater range at the cost of hitting fewer targets.
Mk2 Theurges are... literally unchanged from the mk1 version posted so long ago.
A Trio of Subclasses: One new subclass for each of the Monk, Paladin and Ranger. The Monk gets the new Way of the Drunken Master, the Paladin gets the new Oath of Redemption, and the Ranger gets the Monster Slayer, a tweaked version of the Monster Hunter subclass released for Fighters in Gothic Heroes.
April
Starter Spells: An assortment of new cantrips and 1st level spells for all of the casting classes in 5e.
Downtime: New rules on how to handle what PCs do during downtime in between adventuring encounters.
Feats for Skills: An assortment of new feats based on skill proficiencies.
Feats for Races: An assortment of new feats restricted to characters of specific races.
May
In this month, Unearthed Arcana returned to being a once-per-month production.
Revised Subclasses: Five 2nd edition revamps of subclasses from the past year; the barbarian’s Path of the Ancestral Guardian, the bard’s College of Swords, the fighter’s Arcane Archer, the monk’s Way of the Kensei, and the sorcerer’s Favored Soul.
June
Revised Class Options: More tweaked 2nd editions of subclasses from earlier Unearthed Arcanas; the druid’s Circle of the Shepherd, the fighter’s Cavalier, the paladin’s Oath of Conquest, and the warlock’s Celestial (formerly known as the Undying Light) Patron. It also has some new Warlock invocations.
July
Greyhawk Initiative: Old-school styled rules for handling initiative amongst PCs, using random dice rolls made in each turn of combat. Pretty much universally held up as the absolute worst UA that WoTC has put out to date.
August
Three-Pillar Experience: New rules for gaining experience, in an effort to make exploratioen and social interaction as important to a game as combat, because traditionally all your XP comes from killing shit and that only really encourages murderhobos.
September
Race Options: Eladrin & Gith: New racial rules for the Eladrin, the Githyanki and the Githzerai, with the latter two being presented as subraces of a singular gith race.
October
Fiendish Options: New subraces for tieflings based on the Archdevils, new monster powers and other tips to represent diabolic and demonic cultists.
November
Elf Subraces: Four new subraces for the elf; the Avariel, the Grugach, the Sea Elf, and the Shadar-Kai, in an attempt at amalgamating its 3e and 4e fluff.