From the Profaned Capitalbonfire, climb down the interior ladder and exit through an opening on the wall. Enter the building on the right and follow the hallways until arriving to the toxic swamp.
Head toward the right part of the ruined chapel found in this location and climb up the ladder. Follow the tiled rooftop to the left and enter another opening on the wall which leads into a chamber with a sleeping Monstrosity of Sin and a Mimic.
The complete set can be found as loot in this location.