The realities of the Domains of Dread seem strange or impossible to individuals from other worlds, but for those who live among @The Mists, they're facts of life. With no basis for comparison, domain inhabitants call those who speak of other worlds liars. For them, the following topics are standard aspects of life in their home domain or in a broader collection of domains they call the Land of @The Mists.
Each domain boasts its own culture, either drawn from the Material Plane or a parody manufactured to torment a Darklord. As such, a domain might exhibit traditions and technologies unheard of in other domains. The specifics of each domain's technological advancements are left to the DM. If the DM wishes domains to feature cutting-edge weird science or inventions such as firearms (@Pistol, @Musket, @Blunderbuss, etc), feel free to do so. Regardless of a domain's culture and innovations, @The Mists prevent knowledge of them from spreading. Even if an innovation is taken from one domain to another, the suspicion of the new domain's inhabitants prevents it from gaining acceptance. Such is the subtle control of the Dark Powers, ensuring their nightmare realms remain just so.
Many domains mint their own gold, silver, and copper coins. Though these currencies bear different markings, merchants aren't particular about the designs stamped on coinage. A gold piece from @Barovia spends as well in Borca as it does in Har'Akir, as long as it weighs true. Platinum and electrum coins rarely circulate through the domains, but they appear often enough—originating from hidden troves or ancient vaults—that no trader questions their value.
By impossible coincidence, all domains share a language despite their profoundly different origins: Common, which functions as a shared tongue throughout the Land of @The Mists. Beyond this, all other languages are spoken among the Domains of Dread, some more pervasively than others. The same language spoken in multiple domains might bear subtle differences or unique words. Speakers from different domains might also have distinct accents, but their words are understandable.
DM Option: Domain Languages. If the DM wants to highlight the differences between domains, they can do away with Common and decide that the inhabitants of each domain speak their own unique language, which either take the name of their domain or a culturally specific name. For example, the language of @Barovia could be Barovian or Balok. The languages of specific races, such as Elvish or Dwarvish, remain the same in every domain. Those who wish to learn a domain's language can do so using the training downtime activity. Although it takes 250 days to gain mastery of a language, consider allowing characters to navigate basic social interactions—such as asking for directions or conveying peaceful intentions—after 7 days of training.
In many domains, locals maintain chilly relationships with aloof deities, knowing "the gods" only through hollow rituals and clergy with scant supernatural powers. Conversely, some people privately worship ancestral gods—deities of their family's tradition with whom they form deep, personal connections. Divergent faiths abound, and some that begin as charlatanry inexplicably gain the power of true faith. Ultimately, any deity from any other setting might find followers among the Domains of Dread. By the same token, the Dark Powers breathe life into the beliefs of cruel practices and the faiths of false zealots.
One noteworthy exception to this is the worship of the god Ezra, which has its origins in @The Mists (see "Ezra, God of @The Mists" below). For the DM's adventures, they define what deities are worshiped in a domain and whether those deities are actual gods, manifestations of the Dark Powers, or one masquerading as the other.
The denizens of several domains worship an aloof god known as Ezra. Depicted as a vague, vaporous figure, the god is known for her dark, billowing hair and for her ability to manipulate @The Mists. Her holy symbol is a sprig of belladonna atop a silver kite shield. Beyond that, her disparate sects of worshipers view her differently—and contradictorily. For some, Ezra is a goodly guardian, while others perceive her as a soul-stealing embodiment of @The Mists. Ultimately, though, her true nature is a mystery. Whether she's a manifestation of the Dark Powers, an aspect of the Plane of Shadow's mysterious Raven Queen, or something else entirely is for the DM to decide. Whatever the case, Ezra's followers, traditions, alignment, and the domains she grants her clerics vary widely. Consider characters who are devoted to Ezra to define the god's role in their domain of origin.
The Domains of Dread don't share a unified calendar. However, in most domains, locals measure time by "moons" rather than months. As a measurement of time, each moon begins on the first night of a full moon and lasts a full lunar cycle. A year consists of twelve moons, or twelve lunar cycles. While domains don't ascribe to a shared history, the populace of all domains inexplicably accept the current year as 735. Some local histories cleave to anomalous dates and methods of tracking years, but these are considered obsolete. No such irregularities exist in @Barovia, though, where year 1 corresponds with the founding of @Barovia by the von Zarovich family.
In some domains, the residents are aware that realms exist beyond @The Mists, but most have little interest in lands beyond their own. The Darklords' obsessions distract them from concerns about the nature of their domains or what lies beyond @The Mists. This preoccupation, along with the lack of shared borders or reliable travel, means that mercantile ventures and military conquests between domains are essentially impossible.
Rare individuals do travel between the domains, such as adventurers or roving Vistani families (detailed in the Vistani lore article). Others who wish to travel from one domain to another might wander into @The Mists hoping to be carried elsewhere, or they can employ Mist talismans (such as a @Barovian Wine Bottle) to guide them.
Due to the danger and unreliability of traveling @The Mists, those few with interests beyond their home domains make letter writing their preferred method of communication. A group called the Keepers of the Feather (detailed in the Keepers of the Feather lore article) oversees a private network of carrier @Ravens that possesses the uncanny ability to navigate @The Mists. These @Ravens deliver envelopes and tiny parcels between private rookeries maintained by the Keepers. Individuals and businesses friendly with the Keepers—such as a village notary or inn—might surreptitiously contract their services, allowing customers to send a letter for 1 gp. These letters must include a destination where another Keeper can receive them, then either hold or deliver the correspondence—with delivery costs an additional 1 gp. Letters take at least one day to deliver. The Keepers of the Feather make no assurances about the safe delivery of letters in their charge, but their services prove relatively reliable. Rumors speak of more expensive services the Keepers provide to select clients, such as delivery to individuals whose whereabouts are unknown or verbal messages relayed by talking @Ravens.