What do you think makes a good videogame secret?
This is complicated. Like, a lot of games such as Doom, Quake, Metroid, Dark Souls, these have a mix of secrets. A lot of the secrets in these games are intended to be found however. The trick is, there’s a “secret language” that the developers establish to convey that there are secrets in various places (you should always check under stairs, and bomb off-color blocks for example). There’s a lot of possible tricks you can do for these. Dark Souls always has illusory walls be a bit inset versus the areas around it. Or they show you what’s beyond the wall so you suspect where there might be one. Or they have that boulder wall that needs all the boulders lined up to be broken, which you can figure out on your first try if you’re clever.
There’s tons of stuff like this to use as examples and doing it effectively requires a little thought and maybe some playtesting. If it’s a secret that is essential to progressing through the game, then a lot of playtesting. The key is, people shouldn’t get it on their first try, but they should always be able to get it.
Other secrets aren’t meant to be found, like a lot of dark souls ones. They’re meant to stay secret. These types of secrets get discovered years or even decades later. When you have secrets like this, doled out in a steady drip, it keeps people coming back to the game for years to come. Melee keeps getting new secrets found (and frequently re-found because we forgot about them) and this helps keep up interest in the game. Here’s one such example from Paper Mario:
A lot of that is changing with the advent of data mining, and secrets are being found out more quickly than ever the instant code gets into the public’s hands. Some games like Undertale, and other things like The Stanley Parable, are aware of this, and have intentionally placed easter eggs in the code and debug areas to call out people who try to look too closely at how the game is made.
http://steamcommunity.com/app/221910/discussions/0/792924952700094621
http://vignette3.wikia.nocookie.net/thestanleyparable/images/c/cc/Thestanleyparable-materials-stanley-loadingdock.png/revision/latest?cb=20160126120604
http://thestanleyparable.wikia.com/wiki/Easter_Eggs_%26_Secrets
Or there’s this case where the guy points out clear contradictions between the “lone amateur” story and the way that various parts of The Beginner’s Guide were built.
http://www.blog.radiator.debacle.us/2015/10/the-beginners-guide-by-davey-wreden-et.html
So in order to keep up with this new wave of data miners, you need to be even smarter about hiding secrets in your game. This means using encryption, obfuscation, or other means of hiding information.
What are the secrets that “aren’t meant to be found” in Dark Souls? The most esoteric thing I can think of is the Darkwraith covenant, but that can be accidentally found just with a bit of sequence breaking.
That is one of them. I didn’t mean literally never meant to be found ever (sorry for being vague). I meant more, not deliberately sitting there as something that an average player is intended to find as a part of a normal game experience (like a lot of metroid secrets are).
Also the tree in the darkroot garden would be another good example, or the warp zone in super mario bros (though this one is super popular by now). Zelda 1 also had a huge number of secrets to it, and so did Ducktales and many other NES games.
The idea is that these types of secrets aren’t supposed to just be found as a normal part of the game experience, they’re supposed to be esoteric and found through players pooling their efforts together.
A good point of reference for games that are entirely about these types of secrets would be ARGs, Alternate Reality Games.
https://en.wikipedia.org/wiki/Alternate_reality_game
Celia Alexis Wagar's CritPoints 
