MAW (THE KINGDOM OF THE DEAD)
1) The stone is placed in the window of the ruins between the Maw and the Spine. You can enter the ruins from the west (south-eastern part of the Maw).
SOUL ARBITER'S MAZE (THE KINGDOM OF THE DEAD)
1) The stone is on the seventh level of the Soul Arbiter's Maze, in the last room which has the portal that takes Death to the eight level. I strongly recommend grabbing it the first chance you get, so you won't have to go through the whole level again.
BONERIVEN (THE KINGDOM OF THE DEAD)
1) You can find the stone in one of the first caverns of Boneriven, which you can access by taking one of two paths, and in which you need to use shadowbombs. The walkthrough of Boneriven and how to get there is under the Additional Locations chapter, in the Sentinel's Gaze - Exploring Sentinel's Gaze section (you need to go through the Sentinel's Gaze dungeon).
SERPENT'S PEAK (THE KINGDOM OF THE DEAD)
1) The stone is on the stalactites along the path leading to the bell, prior to the chamber where Vulgrim runs his store.
GILDED ARENA (THE KINGDOM OF THE DEAD)
1) The stone can be found by the stairs leading to the Gilded Arena (enter from the Eternal Throne, not from Leviathan's Gorge).
2) You can stumble upon the stone while exploring the eastern part of the Gilded Arena, on the ceiling in the chamber where you have to fend off Skeletons.
3) The stone can be found while exploring the eastern part of the Gilded Arena, in the cavern where you obtain the first Animus Stone. It's attached to one of the cages.
PHARISEER'S TOMB (THE KINGDOM OF THE DEAD)
1) You can find the stone in the south-eastern part of the lift shaft (the one used to move between the floors of the tomb). Go to the default fourth floor.
JUDICATOR'S TOMB (THE KINGDOM OF THE DEAD)
1) You can find the stone in the southern area, in the oval room which serves as a battleground for two boss fights - with the Bone Keeper and the Bone Giant.
PSYCHAMERON (THE KINGDOM OF THE DEAD)
1) The stone is placed above the door leading to the large cavern where you fight Basileus and Achidna. Keep in mind that you need to face the door from the south (turn 180 degrees after going through them).