See ((C110358)).

The world supertype part is me being silly, but I think that an Era ending conclusively has a better feel to it and prevents buyback-looping.

Maybe an Era should be just a subtype of enchantment that can only enchant a world (whatever that is) and it would say "enchant world" before anything else. This being a world means you can't control a world and an era at the same time and that's a bit weird.

If eras can be referred to by other cards, making an enchantment subtype is probably the best play

@fblthp world enchantments are already a thing from the early days of magic

Ah okay then. So world enchantment is just the term they use for enchantments that effect all players. I was confused because of the rule about controlling more than one world permanent. I thought you might have had some kind of world cards in your set like the planes in planechase.

World enchantments are weird. They were originally called "enchant worlds" (link), although they didn't really attach to anything, unlike auras. When enchantments got cleaned up and "enchant X" (as a card type) all got unified as auras, "enchant worlds" got turned into "world enchantments." "World" now is a supertype like "legendary," and whenever a "world" permanent enters the battlefield, any existing world permanent must be sacrificed

In the early days, there was also the terminology of "local" versus "global" enchantments. Local enchantments all turned into auras, and global enchantments are what you'd think of as "normal" enchantments today

Planeschase is actually based on an early format where you'd shuffle all the enchant worlds as well as a few symmetric global enchantments together, and basically just swapped which was in play at regular intervals