Forgotten Secrets is pretty far away from Drezen but the tools for solving the puzzle are just outside the Commander's fort in this puzzle.
Pathfinder: Wrath of the Righteous has quite the puzzle chain if you are looking to complete the Heart of Mystery and one of the steps is the Forgotten Secrets. Just like all the other puzzles in the chain, you need to loot a batch of slabs and then line them up properly in the cave.
You can find the slabs in the Shrine of Sacrilege, which is located just south of Drezen, past the Lost Chapel. There are some minor obstacles on the way but to avoid spoiling too much, just go in and loot everything - it's really hard to miss the slabs since the map is rather small. You can also see its location on the image above, placing it south of Lost Chapel and East of Greengates. It appears when you move your party in the vicinity.
As for the puzzle itself, it is located far to the southwest but to avoid an annoyingly long journey, you can upgrade some of the nearby forts and then build teleportation circles. Alternatively, you can go to the Heart of Mystery and take a portal to Forbidden Secrets.
When you get there with the slabs just line them up like it's shown on the image below and it's done.
Loot the nearby chests and try solving other puzzles in the chain. Keep in mind that Legacy of the Ancients can't be completed in this Act so you will need to progress the main quest beforehand.