The Destiny 2 Avalon secrets are little puzzles hidden throughout the new Vexcalibur exotic mission that reward you with enhancement prisms, upgrade modules, red border weapons, and even give you a small dose of lore about Asher Mir and what he's up to these days. Unlocking these secrets also rewards you with unique triumphs in the Season of Defiance node.
You'll need to find the Substrate secret as part of the Vexcalibur Expert Authorisation Code quest, which gives you access to the first intrinsic upgrade for the glaive. If you're still exploring Neptune, there are also Neomuna action figures to find and Strand guns to grab from Terminal Overload. Otherwise, here are all of the Destiny 2 Avalon secrets so far.
Data Retrieval: Substrate
This first secret is the one you need to get the Vexcalibur intrinsic upgrade and to complete one of the steps in the Vexcalibur Expert Authorisation Code quest. After you drop into the Vex milk canals, use Vexcalibur's shield to get through the first barrier, jump up onto the bit sticking out the wall on the left, then jump up to the green switch near the top of the room and activate it.
Keep heading through the level until you get to the Vex milk canal with the two harpies, then look up to see a barrier and a ledge. Jump up, use the Vexcalibur shield to pass through, and press the green switch inside. A chest will appear, granting you the intrinsic upgrade. Be sure to open this again on your first weekly run of Avalon, since it will give you a weapon pattern instead.
Data Retrieval: Aberrant Subject
This is the second Avalon secret you can find. After the Vex milk canals, you drop down a long way to a small area with a hobgoblin behind a barrier, and another drop nearby that leads into Variant Security Protocol. Don't drop down just yet. If you look back up from where you dropped initially, there's a green switch just above the doorway you can activate. Now, head into the circular room with the hobgoblin and activate the switch above the other side of the entrance. This makes the barrier and the hobgoblin disappear and gives you a chest.
Data Retrieval: Disposal Subject
This one happens just after the first encounter, where the goblins and minotaur are, but before you crawl through the tunnel and drop down across the disappearing platforms. Look down to the left of the platform to see a green switch you can activate. This will make a Vex conflux appear with shapes that you can shoot. To find the correct order, jump over to the right side, where you can see ledges going up a sloped wall.
Climb these—being wary of the one that pops in and out at the top—then jump right and around the corner to the sloped platform below. From here, you can see another platform and the code hovering above it. You'll need to use Vexcalibur to shoot the conflux nodes, but you can get infinite special ammo from the goblin who spawns nearby. Once you've done that, the chest will appear.
Data Retrieval: Cannonball
After the first boss—second encounter—and the initial jumping puzzle, you'll get launched across a huge distance to another platform, and there'll be step-like platforms sliding in and out of the wall below you. Turn around and look up to see a Vex node. If you have the Expert Authorization Mod slotted, you can use Vexcalibur to destroy this. The next one is floating just off the side of your current platform, to the right, and the last one is concealed in an alcove just to the left and above where the goblin spawns, by the moving platforms.
Data Retrieval: Hard Data Deposit
Hard Data Deposit"
This secret is after the first encounter and the Disposal Subject, but before you drop down across the appearing and disappearing platforms. Instead of jumping down, head right around the corner and use Vexcalibur to pass through the barrier you find there. Now, pick up the orb inside and jump down across the platforms to deposit it in a handy orb recepticle. This secret is also good because it creates a portal you can then take to the start of the first boss.
Data Retrieval: Quarantine Subject
After the first boss, you'll find this secret just by where you have to crawl through the section with the blocks that try to push you off into space, but before you jump into the portal that launches you across the map. Turn right after the blocks to find a green switch to activate, then head back through the blocks to find two Vex in different cells. Pass through one using the Vexcalibur shield, shoot the triangular node inside, and wait for the chest to appear. This secret gives you the second intrinsic upgrade for Vexcalibur.