Noclip's Arkane documentary contains a treasure trove of new details about the development of Dishonored and Prey, but the studio also spills the beans on some projects that never saw the light of day, including its Half-Life spin-off, Ravenholm.
While Ravenholm has been referred to as a Half-Life 2 episode before, Arkane considered it a standalone spin-off that was distinct from the adventures of Gordon Freeman, not unlike Half-Life: Alyx. It featured a different protagonist, too, but not an unfamiliar one.
Adrian Shephard from the Opposing Force expansion would have returned, along with dishevelled priest and now mad scientist Father Grigori (the overeager subtitles translate it to Gregory, but it's clearly Half-Life 2's Grigori), who has developed a penchant for milking and experimenting on headcrabs. Arkane also threw in killer monkeys, zombies and drugs made out of headcrab juice. It sounds wild.
Nine or ten missions were designed, following Shephard and Grigori as they solved puzzles to bring power back to the part of Ravenholm where the priest was living. Electricity features a lot in the gameplay footage, and Arkane says it was a prominent mechanic that would let you solve puzzles and kill enemies. You can see Shephard making a power network using a nail gun and an active outlet, and that same trick could be used to make a trap for the less-than-bright walking corpses.
As well as a nail gun, Arkane created a leafblower, which would have been used to double jump, and even a chainsaw. Weapons were designed to have multiple functions.
While the footage is from an alpha build, it already had voice acting and lengthy scripted sequences, like Grigori making a delicious headcrab stew. He's a bit of an odd duck, using his headcrab experiments to design drugs. Though not in the build shown off, he would have eventually transformed into a monstrous mutant hopped up on crab juice.
Aside from this pair of unlikely allies, Ravenholm didn't have many other characters—just lots of zombies and mutated monkeys that are afraid of the light. Arkane designed acid-spitting zombies and hordes, and while the zombies ain't too smart, you can see them interacting with the environment and ripping off doors. This was before Left 4 Dead, too, when zombies weren't quite as played out.
While Dishonored and Prey are full of big open spaces ripe for experimentation, Ravenholm was more linear. It was still an immersive sim, however, but presumably with a more Half-Life-like structure.
Unfortunately, it wasn't meant to be. According to Arkane founder Raphael Colantonio, it was hard for the team to accept. They put together everything they'd made and sent it along to Valve in the hopes of changing its mind, but it didn't work. Colantonio believes Valve cancelled it because it didn't make financial sense to keep going. It was still a year out from completion and would have needed more resources, so Valve cut its losses.
It's a fascinating look into a game that never was, though it's bittersweet, obviously, because we'll never get to play it. That said, some of it exists within Dishonored and Prey, so you can just go and play them. They're very good.