MAW (THE KINGDOM OF THE DEAD)
1) You can find the stone within the ruins in the north-western part of the Maw (screenshot 1). Jump down to the lower level, go inside the ruins, turn around and look up.
SPINE (THE KINGDOM OF THE DEAD)
1) You can find the stone as soon as you go through the small ruins connecting the Maw with the Spine. When you exit them (which is when you enter the Spine), turn right and make your way up where shown on screenshot 1. Reach the upper balcony. From there you shouldn't have any problems locating the secret.
2) You can find the stone in the large tower to the south-east (screenshot 1). Go up all the way to the top. Make sure you're on the top balcony - the secret will be right in front of you (screenshot 2).
SOUL ARBITER'S MAZE (THE KINGDOM OF THE DEAD)
1) The stone can be found in the first chamber of the Soul Arbiter's Maze, before the room with the tome and the portal. It's hidden by the ceiling, so the best way to shoot it is by standing on one of the balconies.
LAIR OF THE DEPOSED KING (THE KINGDOM OF THE DEAD)
1) The stone is hidden on the lowest, fourth, level, above the door leading to the mausoleum (opened with the Bone Key received in return for collecting a specified number of the Book of the Dead pages).
SENTINEL'S GAZE (THE KINGDOM OF THE DEAD)
1) The stone is lying on the statue standing near the highest balcony of the main structure of Sentinel's Gaze. It's the same balcony which adjoins the room with the hole leading to Boneriven.
GILDED ARENA (THE KINGDOM OF THE DEAD)
1) You can find the stone while exploring the eastern part of the Gilded Arena. After finding the first Animus Stone, return to the south and take the previously inaccessible eastern passageway (screenshot 1). Stop when you reach a small fork in the road, and face north. On the wall you can also find the symbols connected with the Soul Arbiter's Maze side quest.
PHARISEER'S TOMB (THE KINGDOM OF THE DEAD)
1) You can find the stone while exploring the third level, after using the Skeleton Key and finishing the long climb. Stop in the last room, with a statue (eastern-most).
JUDICATOR'S TOMB (THE KINGDOM OF THE DEAD)
1) The stone is hidden in one of the first chambers, the one where you need to push the large pillar and use the help of a Dead Lord (or the Soul Splitter ability). Look up - the secret is attached to one of the cages hung here.
CITY OF THE DEAD (THE KINGDOM OF THE DEAD)
1) You can find the stone while going through the western part of the City of the Dead, after reaching the area where you fight a Wraith (screenshot 1). When the battle is over, follow all the instructions detailed in the City of the Dead walkthrough, specifically the Explore the City of the Dead - return to the western part section. To be more specific, you need to use one half of the soul to reach the area with a chest (screenshot 2).
Look up to locate the secret (the above screen). Try not to get too far from your physical form so as not to end the effects of Soul Splitter too early.