Ars Magica is a pen and paper RPG first published in 1987, which makes it fairly oldschool. It is currently in its fifth edition. The setting is another pseudo-historically accurate depiction of High Middle Ages Europe but this time with MAGIC and WIZARDS, so that makes it totally unique! In fact, the game setting and geography is so uniquely, accurately European that it might as well be called Europe, but instead they just called it Mythic Europe! In order to help hammer that "authentic" Old World feeling directly into your skull, the game is quite liberal with using medieval Latin for a number of game related terms.
Ars Magica was among the first RPGs to use a "troupe" approach to roleplaying. As opposed to having a single GM responsible for all story-telling and control of NPCs, multiple players could instead share the burden, taking up the mantle of "Story Guide" at different parts of the adventure, or run their own side-stories, or even take on the role of story adversaries when appropriate. Given the nature of this narrative and playing style it was not uncommon for a single player to have multiple other characters spread out between multiple stories, sometimes simultaneously, and maybe even in opposition to some of that player's other characters! Although more recent editions did begin to de–emphasize this troupe-style play in favor of the more traditional mono-GM, the fifth edition rulebook does still have rules for troupe play, though they are stashed at the back of the book as an optional rule.
A player's main character in Ars Magica will generally always be a mage, and the multiple magi in the party will all be members of the Order of Hermes. The Order of Hermes is like the United Nations for wizards, which was formed after the fall of the Roman Empire to try to put the kibosh on the sort of destructive and retarded world-breaking things that wizards are generally known to do everywhere, all the time in other settings. Thus the Wizard's Union that is the Order of Hermes pretty much monopolizes magic by killing any "gifted" they come across who refuse to submit to their authority. The Order of Hermes is divided into thirteen different Tribunals, each which oversees a different geographic region of Mythic Europe. The Order is also divided into twelve separate Houses, each representing a distinct magical lineage. And with myth and wizards being the game world's defining feature -since without them, it would just be Regular Europe- it is no surprise that the magic system of Ars Magica is fairly well fleshed-out. Hermetic Magic consists of 15 Arts, which is divided into 5 Techniques and 10 Forms. "Gifted" users of magic can thus manipulate the world around them by using techniques to verb something's nouns. Basically, Five By Ten Magic. There are actually several systems of magic in-universe, each practiced by a different type of wizard, but the Hermetic one just mentioned is usually top-dog for lore and mechanical reasons, and only people who practice it can join the Order, those that don't are usually regarded as beneath notice.
Individual Magi themselves will often congregate with one another in special outposts called Covenants. Here the magi live, study, research and engage in other wizardly pursuits. Also living at the Covenant will be a number of Companions. These Companions will often also be player characters; they are usually "mundane" (unable to use magic), however they have other skills which make them useful in different ways that magi, aloof in their pursuits of magic, will generally have less time to dedicate to, though some Companions can use some of the other, weaker types of magic available in the setting. Lastly, there are the grogs. Grog is a catchall term, akin to serf, for members of the peasantry who perform low-skill and unskilled work at the Covenant. The Covenant will generally have many grogs, and many grogs is referred to as a "turb" of grogs. Grogs are generally treated as shared characters between all players at the Covenant, as viable rules for playing as a grog did not appear until third edition. Because of the many possible player characters and all their comings and goings as they adventure throughout the Tribunals of Mythic Europe, it is often joked that the main character in a game of Ars Magica is actually the Covenant itself. In fact, the covenant is such an integral part of the setting that the rules encourage the story-guide and players to treat it as a character in its own right, with a backstory and 'personality' of its own that develops along with the characters.
Ars Magica was developed by Jonathan Tweet and Mark Rein·Hagen, and first published by Lion Rampant in 1987. A second edition would be published before Lion Rampant would merge with White Wolf Magazine to form White Wolf Publishing in 1991. In 1994 the game was sold to Wizards of the Coast, who would go on to create the fourth edition two years before selling the game's rights to Atlas Games in 1996. The fifth edition was released in 2004; it was the most substantial overhaul the game had received to date, and made many long-overdue improvements to the combat, experience, and character creation process.
The Houses of Hermes:
Bjornaer: House Bjornaer are a crazy animal cult who believe that everyone has an inner beast which you can awaken via a special ritual. They get access to a Heartbeast form, which is an animal form they can take without having to use magic, and can unlock a better form later.
Bonisagus: House Bonisagus are a House created by the founder of the Order of Hermes, and are notable for having several little bonuses, including the right to take other magi's apprentices. They're divided between Magi Bonisagi and Magi Trianoma, who are respectively researchers and peacekeepers.
Criamon: House Criamon is a weird one. Their beliefs are too complicated to summarize easily, but basically, they're crazy Buddhist/Gnostic wizards who think everything that makes Medieval Europe what it is to be evil. Also, they can do things like pass through solid objects, dance objects into their platonic forms, disintegrate objects with their mind, and even ascend to a higher dimension of reality. Told you their beliefs were hard to explain.
Ex Miscellania: House Ex Miscellania is a House mostly of weird misfits who are either barely Hermetic, too weird for the other Houses, or otherwise don't fit. They barely associate with each other and usually don't give a shit about the Order's politics.
Flambeau: House Flambeau is all about killing things (ironically, killing things is the most boring thing you can do in Ars Magica), usually with fire magic. They also do other things, such as academic study of fire, hosting fun tournaments where they literally hurl magic at each other until someone's magical protection breaks, and playing chess.
Guernicus: Called House Quaesitor in older editions, House Guernicus is mostly specialized in law, politics and rules, although they have a few cool tricks, such as old Roman/Egyptian rituals, better Terram magic, and lawyers. Many of them become Quasitores, internal police for the Order.
Jerbiton: House Jerbiton are magi who still maintain fairly active mundane lives due to frequently not being as insufferable as everyone else. They love beauty, art and stuff we on /tg/ only consume once in a green exploding moon. Interesting, but aside from some Mentem and Imaginem tricks don't contribute much.
Mercere: House Mercere is a weird one. They are frequently non-magi, as well as glorified package delivery guys. The few of them that are magi ironically have the best toys, as they get special new ways to use their spells, better portals, and more potent Muto magic.
Merinita: House Merinita used to be nature hippies, but for very complicated reasons involving a guy with gemstones for eyes, murdering a founding member of the Order, and a civil war, are specialized in faeries. They get surprisingly cool toys, such as turning into a faerie, turning into a nature spirit, creating solid illusions, and new ways to build spells.
Tremere: House Tremere are not the vampires most know, instead, they are a strictly regimented House with a death grip on Transylvania. They act like a military hierarchy, and lobby for more organization in the Order. They also are responsible for the most controversial rule in all of 5th Edition Ars, which basically locks them into one schtick, that being magical duelling.
Tytalus: House Tytalus is basically the world's worst family. They love arguing, fighting, and conflict, and hate fun. They are known for literally falling in love with their worst enemies, being terrible foster parents, and killing a lot of people. They also get access to special summoning and healing magic, although the latter turns you into a leper.
Verditius: House Verditius is all about making stuff. Even though they can't cast magic without a wand like Harry Potter expys, they get access to building a literal robot, cursed magic devices, and even more stuff!
The Magical Arts: These are the cool toys you get to play with as a member of the Order of Hermes. They are divided between Forms (what you can do) and Techniques (what you can do it on). One or more Forms are always combined with one or more Techniques. Not included is the non-canon form of Aether.
Creo: The form of creating things. Can be used for offence or healing, but always makes things better or creates them entirely. Requires vis, or magical energy, to be permanent which severely reduces the usefulness of magical healing.
Intellego: The form of looking at things. Usually considered to be the least useful form, it can still be immensely useful for tasks such as finding vis, detecting magic, and looking for lost loved ones.
Muto: The form of changing things. Usually very niche, but can be incredibly potent, such as when changing an opponent's weapon into a stick or changing water to wine. Can't be permanent by any means.
Perdo: The form of destroying things. Interestingly enough, this is the only form that's usually permanent, although it also can't be used to make anything better without combining it with another form. Surprisingly not always associated with killing things.