Mystara, also known as the Known World, is the default setting for the Frank Mentzer version of Classic Dungeons & Dragons (the Basic, Expert, Companion, Master and Immortal sets, and the Rules Cyclopedia by Aaron Allston, which compiled the rules from the first four sets into one volume). It was also the setting of the Capcom D&D games and the Red Steel sub-setting.This setting comprises both the "Known World," the regular world that adventurers start out in, and the "Hollow World," a mysterious, gigantic subterranean world that the characters can explore later on in their careers. Probably the biggest difference between this setting and other D&D settings (apart from its cosmology) is where the clerics of the setting get their spells. Clerics serve one of the Immortals, mysterious and powerful beings that serve the five Spheres of Power, hoping to either expand the influence of one of the spheres or maintain the balance between them. When Immortals meddle in mortal affairs, it is indirectly, and often through some kind of avatar. As characters in the setting reach the pinnacle of power, they have the chance to become Immortals themselves.Mystara was the setting that most people of the 80s who got into Dungeons & Dragons were familiar with, with such familiar faces as Morgan Ironwolf the fighter, Sister Rebecca the cleric, Silverleaf the elf, Frederik the dwarf, Black Dougal the thief, Larry Elmore's beautiful cleric Aleena, and Bargle the Infamous, the bastard of a magic user who murdered her and who every player of that day wanted to take down.There are also some video games for the setting, notably two Beat Em Ups by Capcom: Tower of Doom and the sequel Shadow Over Mystara.
The setting features:
Airborne Aircraft Carrier: In an adventure ideas section of the Dawn of the Emperors boxed set, one suggestion is exploration adventures by combined forces of the two main Empires, with Thyatian flying cavalry operating from Alphatian skyships, and some page art depicting the concept. Serraine is a flying city hosting its own airfleet of WWI style wooden planes; both are powered by gnomish Magitek.
Alternative Calendar: The After Crowning calendar and the Alphatian Year, plus seperate calendars for the Dwarves, Halflings, Ylari, Minrothadi, et cetera. Mystara's year has 12 months of 28 days each, no leap years, and solar and lunar calendars in perfect sync.
Big Screwed-Up Family: several, most prominently the d'Ambervilles, the di Malapietras, the Torions of Thyatis and the Alphatian Imperial Family.
The Caligula: Baron Ludwig von Hendriks. Probably Innocenti di Malapietra too, unless there's a seperate trope for the Borgias.
Clark Ashton Smith: His stories provided inspiration for the d'Ambreville family, and part of "X2: Castle Amber", the adventure that introduced them, was based in his fantasy-setting of Averoigne in medieval France.
Cold Flames: The halfling Masters' racial artifact is blackflame: a dark-colored, frigid "fire" that burns inflammable substances and radiates shadow rather than light.
Cool Airship: Alphatian skyships in general and the Princess Ark in particular.
The Empire: Thyatis and Alphatia can each fill this role depending on your campaign. The most straight-up evil example, though, are the Heldannic Knights.
The End of the World as We Know It: Wrath of the Immortals describes a world-changing adventure there's significant upheval concerning how Radiance is used. It also contains a timeline of events, including regional and world-impacting effects (one of which is caused by a doomsday weapon.)
Fantasy Counterpart Culture Mystara is made of this trope. A partial list includes: Karameikos (Balkan Slavic, with Byzantine influence from their Thyatian neighbors/conquerors), Ylaruam, Nouvelle Averoigne (Renaissance France by way of Clark Ashton Smith), Klantyre, Belcadiz, Caurenze (Renaissance Italy, Court of the Borgias-style), Bergdhoven (Flanders), Boldavia (Transylvania), Ierendi, The Northern Reaches and Heldann (Scandinavia), Ethengar (Mongol hordes), The Atruaghin Clans (various American Indian cultures), Thyatis (with the Thyatians proper as the classic Roman Empire, the Kerendans as the Greek-influenced Eastern Empire, and the Hattians as the Germanic Holy Roman Empire), Thothia, and Sind. Many more exist in the Hollow World, Red Steel, and lunar (yes) areas of the setting.
Fictional Document: Many, including Claransa's Travels to the Center of the World, the Nahmeh (Koran-expy), and the Poor Wizard's Almanacs.
Fictional Sport: Alphatians are big-time fans of a team sport called hardball, which is played on a court divided into squares and involves a lot of complicated passing between players.
Floating Continent: Features of the Hollow World whose shadows provide the only night under the eternal red central sun.
Genius Loci: The Immortal's DM rulebook states that the home planet is a Megalith, a sentient planet. It is known as Urt, and is about half-way through its active phase. At the end of that phase, it will shed its outer baggage (water, life forms, etc) and enter a dormant phase.
Possibly Retconned by the Hollow World boxed set, as megaliths are implied to be solid inside, not hollow.
The Magic Goes Away: The radiance is used to amplify spell power, at the cost of permanently reducing the global potency of magic (to punish the Immortal Sphere of Energy). As described in The Principalities of Glantri, 25 years after the start of the campaign will have enough magical drain to prevent spell casting for one day in the year with the drain increasing at certain milestones. This is corrected in Wrath of the Immortals which cause the drain to be redirected to the Immortal Sphere of Entropy.
Civilizations that are re-located to the Hollow World generally lose much of their magic, both immediately and over generations, as some types of spell are impossible in the planet's interior and becoming a spell-caster there requires much greater minimum ability scores.
Special abilities acquired through the use of cinnabryl must generally be forfeited if their users leave the Red Steel region.
The Magocracy: The Principalities of Glantri and the Empire of Alphatia. Also the Magocracy of Herath in the far west. Wendar and other elven realms kinda-sorta count here also, given that all the setting's elves use arcane magic.
MegaCorp: The Minrothad Guilds are organized like a massive medieval-era version.
Massive Race Selection: While most D&D settings have this trope, Mystara has more player character races than just about any other published setting.
Neglectful Precursors: Arguably, the ancient Blackmoor civilization, given their end and the number of their artifacts floating around.
Unless they're from the Hollow World, in which case they're more likely to be mountain shepherds than miners.
Our Elves Are Better: The standard "high elf" archetype is filled by the forest elves. However, the dark elf analogues, the shadow elves, have chalk-white skin and are not innately evil (mostly just suckered by a Manipulative Bastard Immortal). Finally, there are the water elves, who are seafaring merchants.
Our Gods Are Greater: The Immortals are incredibly powerful beings akin to gods that are ascended from mortal beings. They cannot be hurt by any but the strongest of mortal weapons, they are completely immune to mortal magic, and they can create races and even entire worlds if they want to. The only way an Immortal can be killed permanently is in their home plane — and you have to have the Immortal's express permission to enter.
Our Werewolves Are Different: Well, Malachie du Marais is, anyway. And there were some non-standard therianthropes in the Basic rules, especially in the fourth Creature Crucible supplement.
Schizo Tech: Immortal magic causes this in the Hollow World, but not uncommon elsewhere.
Dragon Magazine's Voyages of the Princess Ark series by Bruce Heard, in which Mystara's outlying regions were vastly expanded upon, contained non-stop Shout Outs to everything from Teenage Mutant Ninja Turtles to Star Trek movies to John Wayne.
Solar CPR: The weakening of magic disrupts the Hollow World's light system.
Space-Filling Empire: Done with the first map ever printed of the planet Mystara, which depicted a major hunk of continent as "the Empire of Dorfin IV", and showed the Empire of Thyatis (whose actual boundaries were much less) encompassing the entire "Known World" region. Averted and Lampshaded by Bruce Heard's Voyages of the Princess Ark article series, which revealed this map to be a complete fraud, perpetuated by a Deadpan Snarker who'd named its various Space Filling Empires after his wife, his mistress, and his dog.
Steampunk: Skygnome tech has strong overtones of this.
The Wall Around the World: Skyshield keeps the atmosphere in. It has openings on poles and is occasionally breached, which spawns an enormous tornado until the hole regenerates.