Let's look at a few of the most important realms. There are a lot of realms when you consider all history from the start of time to the far future, but these are places the Tempest is taking a strong interest, so I'll start with their overview.
(20th century of the mortal realms)
The industrialization of the world really picked up steam in the 1800s and rode that wave all the way to 1914. World War I changed a lot of things, and that's usually when heroes talk about MR20 starting. You see, WWI may not have been "the war to end all wars" (not by a long way!) but it did something we still can't explain. Maybe it was a billion artillery shells or 18 million dead, but somehow the physical and spirit worlds got mangled together in certain places at certain times. Those war years are a major time of spirit activity in the mortal realms, and that's just the start of the weirdness for MR20.
Go forward just a little and in 1934 we see humanity start to fight back. In the Great Depression the US government formed the Special Projects Administration. Tasked with investigating the supernatural and hunting down paranormal threats, the SPA is a nexus point for the rest of the century. In fact, the SPA was still around in 1999 when their major operation for the Millennium marks the end of MR20 and the start of MR21.
(21st century of the mortal realms)
With the discovery of Pulse, humanity quickly replaced electricity with Pulse energy in most areas. Silicon computers were replaced by celestium computers. It seemed like nothing could stop humans from reaching out to the stars and finally conquering the galaxy.
Except that humans could stop humans. The second half of the century is the start of the Pulse War and the collapse of civilization. By the end of the century, the Earth was almost completely devastated. It's the low point for humanity but a high point for supernatural activity in the mortal realms. For some reason, MR21 is a realm the Tempest seems to be taking a big interest in, along with MR22.
(22nd century of the mortal realms)
The second half of the Pulse War was a slow attrition, grinding down the survivors until finally someone did something desperate to stop the war. A pair of machines was constructed, one on each pole. When activated, they sucked all the Pulse from the Earth and channeled it into four huge energy screens. The planet's surface was split into quarters, separating the main warring factions.
The rest of MR22 is the story of survivors overcoming the hardships of their particular post-apocalyptic wasteland and starting to rebuild. Isolated from the other quadrants, they slowly developed until the 23rd century saw them become their own societies.
(23rd century of the mortal realms)
Four lands of the 23rd century stand out in particular: Gatherall, Untamed Empires, Wilderlands and Austeria. With the Earth split into quarters, "post apocalyptic" became different things in different places. Gatherall is a cyberpunk bubble in the middle of the ocean. Untamed Empires has technology from the Age of Sail alongside the smashed remains of Pulse War tech. Wilderlands is a post-apoc nightmare.
As for Austeria, we'll talk more about that when we discuss Maelstrom realms. It's an interesting one, for sure.
(54th century of the mortal realms)
Plot spoiler: the Pulse War wasn't the end of the universe. Humanity survived, ultimately reunited, and finally got off the Earth in search of better times on other planets. There's a whole bunch of realms along the way, but I'll mention MR54 because the Tempest has just started sniffing around the Garden of Athena and the star fleets of the Halcyon Project, the two main galactic nexus points for the century.
With the goddess Athena crippled by the loss of her Tide elemental powers, the realm is vulnerable.
MR realms are the mortal realms, right? Well, so are AR realms. These are alternate realities of MR realms running parallel to the Prime Reality. Technically there are infinite AR realms and mostly we stick to the Prime Reality (there's plenty of work to do here), but it's worth mentioning one AR we've nicknamed Titans of the Jurassic. The Tempest has sent teams across to that reality quite specifically. Maybe they want to recruit a squadron of titanosaurs - now that's a scary prospect.