Illusory Walls are secret passages hidden through Dark Souls, that can be opened by hitting them with any damage. Once the secret entrance is revealed, a distinct sound effect is played, and the player is free to continue through the previously walled area into special sections or to uncover special items.
Some illusory walls in the Artorias of the Abyss DLC require a specific light source to be revealed and produce a rythmic sound when standing nearby. Illusory Walls access to additional items or sometimes even entire areas such as The Great Hollow and Ash Lake or a place where you can unlock a summon for the Boss Fight with Manus.
An Alphabetical list of Illusory Wall locations:
Across the bridge on the lowest floor of the rotating staircase by the Darkmoon Tomb bonfire, is a statue of Gwyn that only disperses if the player wears the Darkmoon Seance Ring.
On the same floor as the bonfire in the castle, the wall is located at the back of an unused fireplace in a room full of chests and tables.
In the swamp at the far end of Blighttown is branch leading into a tree. The wall to the left of the corpse reveals a chest.
Behind the chest hidden in the tree is a second illusory wall leading into The Great Hollow.
Chasm of the Abyss (DLC - use Cast Light to reveal)
Following the main path the player will encounter a fork, going right leads to a large mob of Humanity Phantoms while going left will reveal that the floor is illusory.
Continuing from where the player lands after falling through the floor, a younger Alvina will lead the player to another wall before disappearing. This is in fact an illusory wall leading to her trapped friend.
Immediately to the left of the sealed door opened by the Crest of Artorias is an illusory wall leading to a bonfire.
The second four-sided structure's wall, in the lava pit filled with Bounding Demons, facing the center of the ruins of Izalith is an illusory wall leading to a bonfire.
After breaking the seal, at the deep-water front between the elevator to Firelink Shrine and the building Ingward stands on is an illusory wall that reveals a narrow walkway across the deep-water.
Oolacile Township (DLC - use Cast Light to reveal)
On a walkway (that faces the first mimic in the Oolacile Township area) connected to the two-floor structure occupied by Oolacile Sorcerers that contains the sorcery Dark Orb which is connected to the main 'strip' of Oolacile Township to the right (accessible by taking the path where an NPC invades). The wall is 'unlocked' by a light source as alluded to by the developer message close by.
In the large multifloored building on the lower level of the township. Go up the stairs to the top floor until the path dead ends at a stone brick wall. Using a light source will reveal a narrow doorway leading to a chest.
In the underground area, accessed through either climbing down the well or the ladder near the sealed front-gate, is a series of narrow caves filled with various flat-stone walls that are all actually illusory.
Immediately below the bell, located in the room after the area where the Chaos Witch Quelaag boss fight occurs, is an illusory wall coated in egg sacs.
After exiting the tar pit filled with Titanite Demons, at the corpse hanging over a ledge leading to the start of the level is an illusory wall leading to the roof.
Next to the ladder leading to the second bridge control mechanism is an illusory wall guarded by a spike-trap statue.
In the passage outside the room containing the Large Divine Ember is an illusory wall that leads to Patches and the first bonfire in the area. This acts as the exit to the pit that Patches kicks unsuspecting individuals down.