Dark Souls mod plunges the entire game into darkness

Dark Souls' Tomb of the Giants is one of Lordran's most treacherous environments. With only the player's immediate vicinity lit, the area is mostly pitch black—save for the faint glow of the Demon Ruins lava beneath. Notaprofi's Pure Darkness mod applies the same effect to the entire game.

As detailed by Notaprofi on the project's Nexus Mods page, the darkness varies from area to area. And while bosses "may seem too hard" they're "proven to be beatable, and the areas passable." The mod's creator says Pure Darkness will also test your knowledge and memory of the world's layout, and as such isn't recommended for new players.

Notaprofi also offers the following tips:

You have the lantern! Sometimes it can be useless, sometimes—priceless.
Think twice before picking up treasure. It may be more helpful to you, while laying on the ground.
The lantern, even when it doesn't light your character at all, lets you to see quite remote objects in certain areas. You need to aim it and look 'through it'.
Similarly, you can use firebombs.
The exit from the Pinwheel room is on the right front corner. You just need to know it. You won't see the ladder, but you will see the climb request (unless you're blocking with the lantern.)
Target selection (lock-on) allows you to clearly see enemy position whenever it is.

Notaprofi's Pure Darkness mod is compatible with the game's Prepare to Die Edition. More information can be found here.

For a more lighthearted look at Lordran, here's an area of Dark Souls recreated in LEGO Worlds.