I just spent 1 hour going in and out of the dungeon trying different variations, you have to click all 3 notes outside of the dungeon, in the right order, and then it'll work, I also didn't get the disguise when inside of the dungeon, went straight for the stone (part 3/3) right after i cleared the rubble from the path.
Proceed through the dungeon as normal. When you see "Immersed in Death" do not go to the number 1. Head to the spot, clear the enemies and the object is on the left side of the platform.