That secret is a specific location on the Tangled Shore: a Fallen gravesite. It's a bit out of the way and easy to miss. Here's where you need to go to find the grave, which unlocks a new version of the Tangled Shore Strike known as The Hallowed Lair. If you need more info, check out our Bastion guide for a complete rundown on the Exotic quest.
Head to the Tangled Shore and make your way to Four-Horned Gulch. Once there, you're looking for the Trapper's Cave Lost Sector, located in the middle of the area in a trench near its western side. Yo You won't need to fully complete Trapper's Cave to find what you're looking for, but you'll need to keep your eyes open to locate the gravesite.
Head Into Trapper's Cave
Descend into Trapper's Cave and make your way forward. You'll first hit a group of sleeping Cabal War Beasts. Clear them out and keep moving into the next big room. You should find yourself at the top of a waterfall, with a bunch of Cabal Legionaries and Psions hanging out in the area below you.
Clear the enemies and drop down to the base of the waterfall. With the waterfall at your back, look to your left. You should see the smoky blue trail of Corrupted Ether swirling around the area, making a trail to the left wall of the room. Approach the spot, which is hidden a bit in shadow, and you'll be prompted the check the gravesite.
Interacting with the grave will give you a new step in the Bastion quest, awarding you the Altered Chief quest item. You'll also trigger a Scorn ambush, so be careful not to get yourself blown up while you're investigating the grave. Once you have what you need, you're clear to leave Trapper's Cave without moving on to complete the Lost Sector if you so choose.
Return To Saint-14
Once you've found the gravesite, your next step is to head back to Saint-14 and show him what you've found. That'll unlock a new version of The Hallowed Lair on your Director, titled "The Hallowed Lair, Memento." Head into the Strike and complete it to finish the Bastion quest. Check out our full Bastion guide and our Corridors of Time guide to make sure you complete all the steps, gather all the rewards, and unlock all the story for the Bastion quest.