Ill Bethisad (lit. “the universe”) is a shared fictional universe with an allohistorical framework and the worldbuilding project that uses it as a setting, created by a varied international team of fans worldwide.
The Ill Bethisad alternate timeline and world originate in a constructed language project started in 1997 by New Zealand language hobbyist Andrew Smith. The effort was aimed at creating fictional British Celtic languages with a stronger prevailing Romance influence from antiquity. Thus, several languages were created, most notably Brithenig. Other conlang hobbyists gradually discovered and joined Smith's efforts over the Internet, and by the 2000s, there were several similar constructed language projects with a shared fandom. Given that these projects closely shared a similar premise, the conlangers and later fans that joined in decided to create a single, coherent, consistent allohistorical setting for these languages to exist in. And thus, Ill Bethisad was born, as an extension of all the original conlang projects.
Nowadays, Ill Bethisad is one of the bigger, genuinely detailed and better known AH worldbuilding projects. Its contributors insist that Ill Bethisad is more of a “soft” alternate history in terms of causality. It is notable for generally trying to stick with realism, as well as in-depth setting elements (everything from politics and economy to culture, leisure, transport, scientific developments, etc.). As a project, Ill Bethisad grew so big, that it soon required its own dedicated worldbuilding wiki, which was created as a subset of the conlang/conworld-focused FrathWiki. It is still going strong to date (even though it's probably past its heyday).