It's a fan-created "balance patch" of Diablo 2 which aims to increase build diversity and replayability with "as few changes to the original experience as possible," with periodic ladder seasons, constant patches for balance and content, and increased end-game content (such as Path of Exile-inspired mapping).
Apparently it's been going on for five years now and has a large and dedicated playerbase. In addition to adding new build possibilities from skill tweaks/additions and thoughtful edits of previously overlooked unique items (that are now build-defining), there are also QoL improvements such as increased stash space, online trading platforms, and easier muling.
Anyway, the new ladder reset is occurring this weekend and I'm going to give it a shot. Would anyone on TL want to play?