Half-Life: Delta is a singleplayer mod for Half-Life 1 that creator Yuri, aka XF-Alien, has been working on for a dozen years. It's been released on ModDB (opens in new tab) though yet to appear on Steam workshop, while Yuri just seems glad it's done: "it's not so bad, cuz this mod has been in development hell for twelve (was started in August 2009) years. And now it's finally done."
Yuri was previously responsible for the excellent Arctic Incident Half-Life mod, a smaller singleplayer experience, but this is on another scale with around a three-to-four hour campaign. You play an engineer called Nick Farell who's employed at the titular Delta base complex when everything goes wrong at the Black Mesa Research Facility: and nasty alien monsters start crossing from Xen to Earth.
The campaign features 31 maps across four chapters and Yuri calls it a "total conversion" mod: the video and screens show the amount of work that's gone into reconfiguring these environments (see the comparison screen below). It adds new weapons, has its own OST, and new enemies on top of all your toothy favourites.
"[Delta] is my largest and oldest project," writes Yuri. "I'm glad to finish the development. It was 'a bit' difficult to make 300+ models, custom texture wads, a lot sounds and, of course, 32 single maps. Also, last summer I decided to return back some cut content (new NPC with her scripts). The plot was incomplete and illogical without this NPC."
The mod also has voice-acting, but not yet: this will be in the Steam version, whenever that's approved, but the current mod build only has English subtitles.
The Half-Life series has always been fertile ground for modders, and arguably the most impressive of all is the now-finished Black Mesa (opens in new tab), which over the years pretty much morphed from a mod into a semi-sanctioned full release. Valve itself has been quietly updating Half-Life 2 (opens in new tab) recently in preparation for Steam Deck, while modders focus on remastering the entire thing (opens in new tab) (or remaking obscure spin-offs (opens in new tab)). Half-Life's official future is as opaque as ever; the unofficial track, however, continues to deliver.