Board Thread:Dark Souls/@comment-91.120.90.210-20140616222205/@comment-3975833-20140816082111

Blighttown is accessible through the shortcut under Firelink Shrine, the entrance from The Depths, or the Valley of Drakes (which is in turn accessible through Darkroot). The Gaping Dragon and Capra Demon are both optional with or without the master key because of the Valley of Drakes passage.

The Taurus Demon bossfight is optional if you have the master key.

The Demon Firesage and Centipede Demon are optional if you're willing to contribute a bunch of humanity to the Chaos Servant covenant for the shortcut.

Stray Demon, Moonlight Butterfly, Dark Sun Gwyndolin, Crossbreed Priscilla, and everyone from the DLC are always optional.