Dark Souls' interlocked Lordran setting is dangerous, deadly and, as its title suggests, dark. That is, until modders tinker with its brightness and lighting effects.
Spoilers afoot, with regards Dark Souls level design and boss zones:
In the video's description, El Friede explains the above is achieved by removing the DrawParam folder and tinkering with its RGB values. In turn, the game's darkest zones—not least Blighttown, the Tomb of the Giants and The Depths—have an omnipresent glow that makes them somewhat less terrifying than their regular states.
For the sake of comparison, Notaprofi's Pure Darkness mod is the antithesis of El Friede's Bright Souls. It too fiddles with the game's DrawParam folder, but in an obviously different way.
Besides taking the edge off of its most sinister arenas, the likes of New Londo Ruins and Sen's Fortress as they feature there remind me of early Tomb Raider or Silent Hill games. Naturally, these drastic lighting adjustments create some quirks in the few shadows that remain—further underscoring the mod's PSone-style effect.
As also observed by a few Reddit folk in this thread, I hadn't spotted the Flaming Attack Dogs that power Blighttown's elevators until now. Every day's a school day.