Twelve extended families with "dragonmarks" that grant them magical abilities based on a certain theme. The houses act as the Fantasy Counterpart to Mega Corps, controlling all the major guilds related to their mark's functions. Despite there being thirteen official houses, there are currently only twelve types of dragonmark in existence - the Mark of Shadow is carried by two competing houses, while the carriers of the Mark of Death are thought to have been wiped out.Dragonmarks outside of these are referred to as "aberrant marks", granting unique powers which are evil or destructive in nature; most were wiped out in the War of the Mark (which also established the Houses), though weak aberrant marks still pop up on random individuals throughout Khorvaire.
Tropes common to all houses:
Amplifier Artifact: Many magic items created by the houses function by amplifying the powers of their specific dragonmark (or are simply more effective when used by someone with that dragonmark), allowing them to be created much more easily than normal. This is how items such as teleportation circles can exist in settings where high-level wizards are rare.
Feuding Families: Comes up at times when the fields of two houses overlap. The families themselves are large enough that there's more rivalry between regional branches.
Hybrid Overkill Avoidance: While the only mechanical requirement for gaining a dragonmark is being the same race as the house, members of subraces (eg. drow elves, whisper gnomes) are considered too genetically distant from their houses to have had a member as an ancestor. Likewise, anything which changes your race (such as becoming an undead or dragonborn) disables your dragonmark. Cross-breeding two dragonmarked lines just creates an aberrant mark with apparently random powers.
Subverted with Erandis d'Vol (see House Vol below).
A human dragonmarked house with the Mark of Making, who come up with all the magic ideas and are the main reason why the setting has Magitek. Their House's original headquarters was in Cyre, but it was lost on the Day of Mourning. They also run the Tinkers Guild and the Fabricators Guild.
A halfling dragonmarked house with the Mark of Hospitality, they keep your hero sheltered and fed. They run the Hostelers Guild.
Trauma Inn: It's their job. Some of them can even summon inns.
A halfling dragonmarked house with the Mark of Healing, they heal your team when they are down. They run the Healers Guild, which has managed to displace clerics as the populace's go-to source of healing magic (Eberron temples generally only offer healing in emergencies or for those working for them).
The Caper: Their magically-guarded bank vaults seem to exist primarily to enable these.
Dungeon Crawling: They've been building vault complexes/deathtraps for so long that they've lost the details on what some of them are for (or in some cases that they exist at all), requiring them to send teams to investigate every so often.
Dug Too Deep: An ironic example, in that they tend to have put the underground dangers there in the first place, then lost the records.
An unofficial house made up of bearers of aberrant dragonmarks, who sell their skills as thieves and assassins. Their headquarters are in Sharn. The original members came from an experimental aberrant unit in The King's Dark Lanterns (Breland's intelligence agency), who turned resentful after their superiors tried to abort the project. By killing them.
Bad Powers, Bad People / Bad Powers, Good People: while the former are the ones you'll usually encounter in a fight, the House also consists of people who are actually decent people, whose only fault is that they bear the Aberrant Dragonmark.
Combo Platter Powers: Unlike standard marks, the powers granted by aberrant dragonmarks rarely follow any theme other than being destructive.
Sins of Our Fathers: They hate the present dragonmarked houses because their ancestors killed those with aberrant marks in the War of the Mark.
A (now extinct) elven dragonmarked house with the Mark of Death, whose specialties included contacting the spirits of the dead and creating undead creatures. In an attempt to bring peace between the then-warring elves and dragons, they produced a half-dragon elf girl named Erandis d'Vol. They succeeded in bringing the races together... to exterminate House Vol so that that such an abomination could never be born again.Erandis still exists in hiding, her mother having saved her by transforming her into a lich. As a half-dragon her dragonmark is mutated into a unique form with world-shattering power, but due to her being Undead she can no longer use it. At the same time remnants of House Vol in Khorvaire have created a religion called The Blood of Vol, which Erandis secretly controls.
Big Bad: Erandis was designed to be a villain for mid-high levels.
Broken Angel: Erandis is the all that is left of her bloodline. Her lich status prevents her from using her bloodline.
Dark Is Not Evil: What exactly the Mark of Death did isn't clear, but it was something that contributed to society rather than just being destructive.
Last of Her Kind: Erandis is all that is left of her bloodline. Due to being a lich she can't revive it the conventional way either. Pulling off resurrection grows harder the longer someone has been dead, and the house has been dead for a really long time, so that's probably not going to happen. She has tried some experiments to bring her bloodline back, but they have failed. To make things worse Keith Baker has stated that the true nature of the mark will not be revealed, practically guaranteeing any attempts in supplements she may attempt will fail. Some aberrant dragonmarks (whose wielders are reviled on a level to be decided by the DM) might be descended from the Mark of Death, but figuring out which and how would be extremely difficult. All we know from the game mechanic books (including supplements) is that in the fluff Erandis' mom used the mark to turn Erandis into a lich, and even that is stated that it might not be true. There is also what we have in the Matt Forbeck's novel series The Lost Mark.
Not-So-Harmless Villain: Relatively speaking. Erandis doesn't directly control most of the Blood of Vol cult, her actual minions are fewer still, and while pretty high level she's too weak to fight the Chamber or the Lords of Dust directly. However, if she were to find a way to regain access to her dragonmark then her power would skyrocket, making her one of the strongest beings in the setting. Plus Word of God says that fighting Vol in her own sanctum is near-suicide - while she's there she can do insane things like summon Future Badass ghost versions of anyone who attacks her.
Tropes common to all religions in Eberron:
Clap Your Hands If You Believe: Clerics cast divine magic through their faith in a deity, rather than a deity bestowing power on them like in most Dungeons & Dragons settings. This allows for clerics to follow a distorted version of their church's teachings (such as a Chaotic Evil cleric of a Lawful Good religion or vice versa) without losing their powers, and for beings who are not deities to have clerics.
Corrupt Church: Without direct divine supervision, it's much easier for this to happen than in most settings.
The Nothing After Death: Dolurrh, the Realm of the Dead. "It is not a reward. It is not a punishment. It simply is."
The Sovereign Host
The most commonly-worshiped pantheon in Khorvaire.
The Dark Six
A group of evil deities (with the exception of The Traveller) who serve as counterparts and sometimes foils to The Sovereign Host. Commonly worshiped among monstrous races. Even ostensibly good people will also sometimes offer them respect in secret to avoid trouble (sailors appeasing the Devourer to prevent storms while at sea, for instance).
The Church of the Silver Flame
Worshippers of an enormous silver fire which burst from the ground in a battle between a demon lord and human paladin named Tira Miron, assisted by a couatl. The Silver Flame is currently located in the enormous cathedral of Flamekeep, which also serves as the theocratic capital of Thrane. The head of the church is the Keeper of the Flame, a Holy Child with the ability to communicate with it.
Evil Counterpart: The Shadow in the Flame, the voice of its sealed demon, which has its own worshippers and even its own more powerful version of the Keeper of the Flame (who is currently imprisoned in Dreadhold).
Fantastic Racism: The church is historically suspicious of shifters due to The Purge, though many members are apologetic about the whole thing. As a result, shifters who worship the Flame tend to be unusually devout in order to prove themselves.
Field Power Effect: The Keeper of the Flame gains a massive power boost while within Flamekeep. Jaela is an 18th-level cleric inside (possibly the only 18th-level cleric in the world), but only a 3rd-level cleric outside.
The Purge itself was not entirely a bad idea and was prompted by a genuine threat - with Eberron's twelve moons, that's a full moon every couple of days. So every couple of days, just about every afflicted lycanthrope (the ones who can't control their changes or what they do while changed) in the world is going on a rampage.
An ancient religion which eschews external deities, instead focused on reaching "the divinity within" through the power of the worshipper's blood. Its present form was created by remnants of House Vol, with Erandis d'Vol (aka "Lady Vol") as its secret leader. Due to its individualist nature, sects vary wildly in practice and attract members of all races and backgrounds.
Corrupt Church/Religion of Evil: The higher ranks of the church serve Erandis d'Vol, but most members are normal people who don't even know she exists.
The Undead: Becoming a sentient undead is seen as a form of martyrdom in the church, sacrificing your own chance at divinity in order to help others. Animating corpses as mindless zombies, meanwhile, is just pragmatism; they're excellent workers, and it's not like the spirits of the deceased need them any more.
We Are Struggling Together: There are almost NO unifying tenets of the Dragon Below cultists besides claiming to worship Khyber. Combine that with a depressingly high rate of insanity and any two given cults are as likely to be killing each other as they are anyone else.
The Path of Light
A meditative religion practiced by the kalashtar, who seek to change the nature of the universe and usher in a golden age through the power of positive thought.
Time Abyss: Feared by the dragons as one of the few factions who can manipulate events over a longer scale than they can.
The Lord of Blades
An enigmatic warforged warlord who seeks for his race to rule all of Eberron, said to be building an army of followers somewhere in the Mournland (literally; he has access to the creation forge in House Cannith's old headquarters). It's unknown where The Lord of Blades came from, or even whether "The Lord of Blades" is the name of a specific warforged or just a title; some think he was just invented by dissident warforged as a rallying symbol.
Blade on a Stick: The weapon the Lord of Blades is usually shown wielding (with too many blades on the end), though his followers prefer greatswords.
Wolverine Claws: In some depictions. An image of his clenched fist has even been used as a holy symbol.
The Becoming God
A loose group of warforged tribes, found mostly in the Mournland, who feel there is no place for them in the religions of other races. Instead they plan to build a Physical God for their race in the same way they were built. Their clergy are often encrusted in scrap metal and bits of magic items. Their reactions to the Lord of Blades are mixed, with some seeing him as a madman and others as a prophet of their faith. An alliance between the two, propelling him to godhood, is presented as a worst-case scenario.
100% Adoration Rating: He's astoundingly popular a monarch in his own country and even abroad. Partially it is what helped him become the engineer of the current peace. He's so popular that many of the setting's resident chessmasters hone their plans to begin after he dies of old age, because his popularity is such that it's better to not have to deal with it. Incidentally, the fact that none of his sons share his popularity has been noted by many. Even the people who desire to make Breland a democracy realize that their idea has no chance of passing during Boranel's lifetime.