SideQuest - Sentinel's Gaze
(First visit, after reaching the first time)
Before entering the dungeon, head to the left and find a Boatman Coin near the wall, behind a thorn. Enter the ruins afterward.
Head downstairs the stairwell and wall-run to reach a post once you see it. Climb down until you see a chest. Loot it then climb up the post and wall-run to the next set of stairs until you reach the door. Continue forth and destroy all enemies. Get the Boatman Coin in the northeast corner.
Climb the northwest ledges and check the torch near the edge to find a Relic of Etu-Goth. Grab a shadowbomb from the top then destroy the crystals below. Loot the chest and destroy the weapon racks to get some random weapons. You can check the second floor in the main building but you won't be able to fully explore it just yet. We're done for now so just fast-travel to the exit.
(Second visit, after getting Soul-Splitter)
Return to Sentinel's Gaze and climb the second floor of the main building. Stand near the edge where you can clearly see the shadowbomb pod. Soul split and send one soul to grab a shadowbomb. Throw that to the other soul so it catches it. Use that shadowbomb to destroy the crystals.
Climb up and shoot the Stone of Power on top of the statue. The next passageway is blocked by crystals again to soul split again and position one soul at the edge, overlooking the shadowbomb pod. Switch to the next soul and grab a bomb then throw it back to the other soul. Use that to clear the next room and get the chest.
Behind that chest is a hole that you can drop off. Jump in there to reach another optional area.