With the release of the free A New Reign expansion, comes more puzzles and mysteries from Klei.
The Fuelweaver's 3rd form, fightable in the Atrium, has a string of quote that said "Metheus..."
The user @ImDaMisterL, with his ingenious and brilliant mind, just goofed up the webpage and inputted that there as a what if. What an Einstein.
Click here to be redirected to the puzzle's page!
So, what exactly lead the community to think something was going on?
Klei posted the release trailer of the A New Reign free expansion:
And it had some moments where hidden images could be seen i one paused at the right time before/after/in the middle of transitions.
The first one:
That seemed to depict weird symbol, maybe even a language, and the community already started to theorize. Some people linked the some of the symbols to older William Carter/Maxwell puzzles.
The second one:
The First Day
When entering the website, you'd be face to face with this:
Yes, a cave wall and what appears to be some torches/lanterns. Nothing special was happening, and we realized we'd have to wait.
Ater that, the user @Mobbstar used pro hacking skills to access the available assets page and found 4 very interesting images:
Those images appeared to depict the murals that told us a bit of the ancients' story in-game, but much more detailed now, of course. People realized though, that the 2nd mural was missing.
Some time after, Klei restricted the assets page more than before, and if you tried getting those images now, they'd be like this:
The Metheus page updated again that day, adding bowls of what seems to be paint and brushes:
The Second Day
The page updated once again:
@CarlZalph Made a compilation of the symbols and the apparent alphabet of the ancients:
And @Zeklo made a post with each individual symbol, found here:
@Cherryzion made a compilation of the symbols we've had in older puzzles, their names and meanings:
The page updated 3 more times that day:
@Maslak Made a truly great discovery as well: metheus.dontstarvetogether.com/tenebris
The Third Day
In this day, the page updated thrice!
According to @Canal_WP, the page will update 2 more times today, at 2:55 and 10:55 PM, as it appears to update every 8 hours. EDIT: That turned out to be wrong 
The Fourth Day
So far, the page has only updated once today:
Thank you so very much, @Aeschwutz, for helping me with all the images I haven't (and even the ones I have) collected. This probably would've taken much more time if it wasn't for your awesome help!
Huge Update (And link to guide!)
This topic would be updated as the puzzle progressed, but I'm a bit busy nowadays, so I'll take longer to update it properly.