There are all sorts of mods for the original Portal that add new test chambers and puzzles. But a mod called ERROR leaves the Aperture Science facility mostly as-is while still managing to change the entire mood of the experience.
Portal is a hilarious first-person puzzle adventure, but ERROR manages to turn it into a creepy psychological horror.
With the mod installed, Portal feels immediately weird. You don't get the voice of GLaDOS greeting you as you wake up and begin playing: The Aperture Science facility is quiet and dark, almost like everyone's gone home for the day. It's honestly extremely weird to be solving those first few puzzles in relative silence. It feels like you're not supposed to be there at all.
There's more than just a vague feeling of dread. Sometimes the lights abruptly go out completely. At times I'd be stumbling around in the dark and suddenly realized I was somewhere else, a different room or a long corridor with a single, empty chair placed in a harsh spotlight. Id' hear eerie audio cues, distant sounds that may have been someone's voice, or something else behind the walls, before the lights blinked back on and I found myself back in the test chamber again.
And there are more overt horrors. One test chamber was hazy with smoke or fog, and I went about solving the puzzle, not noticing until nearly the end that there was a human body lying in the gloom. I almost walked right over it without even seeing it. Employee? Former test subject? Either way, it was a jolt.
The mod's page on ModDB keeps the spooky narrative going. In a series of blog posts, modder M_K_L, states that ERROR is a corrupted version of Portal they were somehow able to download, and that a mysterious entity called Dead Devs sent a number of foreboding messages warning M_K_L not to release the files.
It's an enjoyable continuation of the fiction. Does ERROR take place sometime far in the future, when GLaDOS is only barely still alive? Is it in the past, around the time she was flooding the facility with neurotoxin? Is it an alternate reality? Who is that empty chair for, you or someone else? It becomes clear early on you may not be entirely alone.
ERROR is a pretty neat mod—it's impressive how effective it is without making major changes to the game many of us know by heart, and it's a fun and spooky way to revisit the classic Portal.