Which Might and Magic universe are you referring to?

Well, maybe my problem is that I only played Heroes.

Obviously, yes. Although there is a lot of lore information in Heroes as well, but, of course, it is far from complete.

Well, maybe my problem is that I only played Heroes.

It didn't seem that way to me. It feels rather hastily and crudely implemented. Some kind of elemental dimensions... Something with a slant towards parascientific concepts.

I agree, but portals can also be criticized. For example, it seems strange that for centuries people lived and knew nothing about portals, and then BAM! - and here is salvation on a silver platter.

It was implemented hastily and somewhat crudely, but this applies to the faction itself: it was only planned for the second expansion pack. As a result, due to a bunch of idiots who wrote angry letters and threats to 3DO, the Forge of the Gods in Armageddon's Blade was canceled, and no new faction was introduced in Shadow of Death.

The Elemental Planes are not "some kind of," but have been present in the series since 1988 - with Might and Magic Book Two: Gates to Another World, and have directly appeared and been mentioned in the games repeatedly. These are parallel worlds, each embodying one of the four elements (Fire, Air, Water, Earth) and inherently opposed to each other. The School of Elemental Magic often interacts with these Planes (the same elemental summoning spells).

Something similar in concept exists, for example, in some D&D universes.

I agree, but portals can also be criticized. For example, it seems strange that for centuries people lived and knew nothing about portals, and then BAM! - and here is salvation on a silver platter.

I agree that this is a "deus ex machina," as they say, but this applies in general to The Reckoning. And the portals can easily be given an organic explanation. For example, they may be connected to the Web of Worlds, repaired shortly before The Reckoning, or be the result of the magical resonance of the Blade of Armageddon and the Sword of Frost, just like the cataclysms that destroyed the planet.