Belfry Sol

This is accessible in the Iron Keep area where there are suspended platforms. You have to use a Pharros Lockstone in contraption behind the heavy iron door, beside the control lever. There's no boss behind the mist however, there are dark dwarven spirits and invader spirits you have to fight. If you're playing online, you'll be pulled to another player's game; if you're playing offline then you'll just have to deal with bots generated by the game. These phantoms don't respawn when defeated.

There are two towers in the corner; one has a lever that will lower the gates to the exit, while the other has a ladder going to the rooftop. There are two phantoms in the initial ground, one in the rooftop and one near the exit. The Dark Spirit Bellkeeper will also invade you after a while.

Find the corpse in the base of the roof leading to the exit. Loot it to get a Simpleton's Spice. Defeat the last two dwarves and loot the last corpse near the exit that also has a Simpleton's Spice. Finally, loot the chest in the exit to get Immolation.

Go downstairs and loot the corpse there to get a Human Effigy and Triclops Snake Troches. Follow the path outside and you'll be back in the Iron Keep, near the bonfire.