I've been playing a bunch of Dark Souls lately after a fairly long hiatus. The game is of course five years old, but I only got it late last year basically because I've become obsessed with its spiritual sequel, Bloodborne, but do not yet have a PS4. Dark Souls is sort of unlike any other game I've played, though I'd call Metroid (well, Super Metroid is the one from that series I've played the most) a sort of antecedent.
Metroid games are all about gradually expanding the areas you can explore. In Metroid, you typically do this by unlocking new tools or weapons that let you get the job done. Dark Souls borrows a lot from Western RPGs (a term that confuses things a bit, since Dark Souls is pretty unambiguously a Japanese game,) which includes the ability to use different playstyles, and so a "gadget-based" exploration system wouldn't totally fit.
So the world gets unlocked usually by means of opening doors and getting keys and such. Interestingly, a lot of the time "unlocking" an area is less about getting there for the first time and more about creating a shortcut so that you can get there more easily. For instance, to get to an (infamous) area called Blighttown the first time, you have to go through an aqueduct, up into the rooftops of the "Undead Burgh," up onto the city walls, down into the lower part of the Burgh, down into the sewers, and finally farther down into Blighttown. If you explore thoroughly, though, you can find a key that unlocks a gate leading to a couple of elevators that get you right back up to Firelink Shrine - the activity hub around which most of the friendly NPCs you encounter congregate.
What's also kind of interesting is that sometimes there's no clear barrier between areas that you are ready for and those you aren't. For example, there's an area called the Catacombs - a massive subterranean burial ground filled with undead enemies. I was able to beat that area's first boss, Pinwheel, at around level 30. But right after beating that guy, you can keep going into an area called the Tomb of the Giants, which at level 52 I don't yet feel ready to take on.
Of course, seasoned players can learn to handle these hard enemies at a lower level. Healing resources are severely limited in Dark Souls. There is one ring you can find (I haven't) that gives you some HP back after killing an enemy, but otherwise, you're limited to healing spells and Estus Flask charges. These get reset when you rest at a bonfire, but so do almost all enemies, meaning that you need to be very conservative with your heals unless you know you can recover very soon.
Because of this, almost every enemy's attacks can be fully avoided. Many shields will prevent 100% of the physical damage a strike would do, though you have to be careful to let the shield down between an enemy's attacks to recover the stamina needed for blocking, as well as striking and dodging.
On the other hand, though, because every enemy's attacks can be avoided, they tend to hit quite hard. Even the skeletal soldiers around the Undead Parish - a pretty early area in the game - will take a good 10% of my health out if I manage to get hit by them. That might not sound like a lot, but when you can only heal a finite number of times (and there sure as hell ain't any health regeneration over time - though I think you can get a gem or something that does that in Bloodborne) every hit you take is dangerous.
It's hard to identify why this isn't just horribly frustrating. I suppose it's that the game feels like it usually obeys its own rules. If you're clever, you can exploit those rules. For instance, in Anor Londo (a late area in the game,) there are nasty gargoyles that do lighting attacks. But they also tend to jump around a lot in areas with no railings. You can sometimes maneuver them to jump off a cliff, instantly killing them. Sure, they have wings that they use for some of their attacks, but if they're doing their backward jump, they're not primed to fly.
Perhaps some of the difficulty is the way that things are left unexplained. For example, the functionality of Humanity (a resource that, like Souls, is left in your bloodstain when you die and has to be recovered) is totally obscure. Also, I'm still not sure I have any idea what the Covenants do. You're encouraged to experiment, but at the same time, there are a whole lot of no-take-backs. For instance, if you kill a friendly NPC, they won't come back (even if they're supposed to be undead like you.)
Still, the difficulty makes success feel that much more satisfying. It's also definitely the sort of game that will give you your money's worth. There is an insane amount of depth here, both in the world and items you can find, the playstyles you can try out, and the story, which is filled with open-to-interpretation ambiguities.
An exploration-based game is all about mystery, and Dark Souls makes investigating that mystery a huge challenge.
(EDIT: For the record, Executioner Smough, you are a dick.)