This is probably the most thematically immersive place in the game. You start out in the undead burg. There's sun, connections to beautiful places, plenty of living characters just hanging around. Then you come across a demon (Taurus) that seems sorta out of place.
Then you enter the lower burg. And as you venture down further, the setting becomes darker and more dismal. Thieves and hounds trying to bleed you. Then another, seemingly, out of place demon (Capra). You come across a couple of humans here, but they're barely alive, and need to be actively saved by the player.
In the beginning of the game, your aim is Blighttown. That's all you know about. So you get there, and it is, indeed, a pretty dark and treacherous place, filled with odd demonic monsters. And then you realize.
That whole path to Blighttown?
Yup, just the halfway point on the descent into the darkness. Soon all of the humans are straight up demons and you're wandering through this eerie and fickle hellscape, where the dangers are all-or-nothing threats.
Could the final area boss have been better? Yeah, sure. But this whole, long path from the burg to the Bed of Chaos is basically the most "Dark Souls" feeling section of the game, and maybe even in the series. That whole concept of a gradually deeper and darker path, with thematically similar enemies, as you progress was almost entirely lost in the second game--where most of the areas felt like disconnected/thematically discrete regions.