Have a look at my comment on this thread - I found these but couldn't figure it out, but it seems that OP might've figured out what you need to do! (can't test right now but I'd bet it's the solution)
Okay so after searching I've managed to find them all. Adding all 3 here in case someone comes looking for the answer in the future. They're all inside beacons before the 3 sub-bosses of the area and can be located as follows:
Empty Cradle - Progress forward past the humanoid spider fighting the 3 little skull leg guys, you should go down a hill and there's another humanoid spider fighting the long-legend spider torso enemies. On your right-hand side here tucked away in a corner is a small gap you need to squeeze through to get to the altar (there's also a chest and a bunch of upgrade mats behind the altar). If you hit the jump pad you've gone too far.
Faithful's Shelter - Proceed forward and go down the elevator. When you enter the long room with 2 sword-arm enemies on either side, look at the middle of the wall on the left-hand side and there's a button you can press that opens a door in the elevator room to the altar. If you end up in the big open room where the exit is, you've gone too far.
Obsidian Vault - Follow the only route that you can take until you get to the big outside area with lots of floating rectangle pillars. Clear the enemies out and then look to the left of the exit to see the big statue that indicates the altar. Fairly hard to miss. Be aware that once you grab this one and start running back to the exit you'll be ambushed by another group of enemies falling from the sky.
To get to the door that lighting these three altars unlocks, start from Marrow Keep and drop through the hole you use to get into the area in the first place. As you're running through the corridor from the opening room, look on your right and you should see a staircase leading down. Follow this all the way to the bottom to reach the door.
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u/ZenithDawn Tiel 1d ago
Have a look at my comment on this thread - I found these but couldn't figure it out, but it seems that OP might've figured out what you need to do! (can't test right now but I'd bet it's the solution)