An Illusory Wall is a type of fake wall that will disappear upon hitting it or using a certain item. Some may reveal secret items or bonfires, or even Covenants and entire areas.
By the bottom of the ladder near the lever that controls the second rotating bridge, an illusory wall leads to a hidden bonfire. This illusory wall is "guarded" by a statue that shoots spikes.
A large treebranch in a far corner of the poisonous swamp leads to a series of two illusory walls. The first is near the corpse containing the Plank Shield, which reveals a treasure chest. The second is directly behind said treasure chest, leading to The Great Hollow and Ash Lake.
After descending the stairs by the lever that rings the second Bell of Awakening, an illusory wall can be found that leads to the Chaos Servant covenant and a bonfire. If you use Seek Guidance, a developer message will point to the correct wall.
By the top of the tall ladder that exits the tar pit, an illusory wall leads to a rooftop above the gate. There you may battle the Giant that opened Sen's Fortress after ringing both Bells of Awakening. It is the only illusory wall in the game which requires you to attack it to disappear instead of rolling or kicking.
Another can be found at the lowest floor reached by the rotating bridge. Equipping the Darkmoon Seance Ring by the Darkmoon Tomb bonfire will remove an illusory wall to reveal Gwyndolin's boss fog and therefore access to the Blade of the Darkmoon covenant.
At the second structure in the lava area, an illusory wall contains a hidden bonfire.
Traveling from the first bonfire to the tomb with the Large Divine Ember, an illusory wall can be found on the wall to your left shortly before reaching the tomb. It is where you exit from the pit Patches kicks you into.
There are several illusory walls underground by the Skeleton Wheels. All of them are easy to see and look different than surrounding walls.
After taking the alternate path that leads to the rooftop with the Crystal Lizard (turn right and walk around the building before the first wooden bridge, follow route and then roll onto the ledge beneath the wooden platforms), follow the perimeter of the structure and stand on the side closest to the Crystal Lizard. Use a light source by the developer-placed message that says "Let there be light…", like a Skull Lantern, Cast Light, or the Sunlight Maggot to reveal an illusory wall.
A second illusory wall can be found in the upstairs area of the large building that contains many Bloatheads. On a wall by the broken floor that requires a jump to reach a hidden treasure chest, you can use a light source to reveal the hidden path. It leads to the same treasure chest, bypassing the need to jump.
If you listen carefully when close to an illusory wall in this area you will hear a slight pounding noise that fades away as you leave the wall.
Near the end of the first descending path by the first group of Humanity Phantoms, there's a fork in the path. Go to the left, the floor will disappear and you will fall below (but will take no damage). From there, follow an illusion that appears to be Alvina. The illusory wall is behind the third illusion. Not far from there is a shortcut elevator leading to the Royal Wood. If you already accessed the elevator and miss Alvina's guidance, just start from the bottom of the elevator and hug the left wall.
Other Breakable Walls
There are also some breakable walls in the game that aren't classified as illusory:
A boulder will break open a wall to reveal Oscar, Knight of Astora.
Breakable boards of wood in the upstairs of the church will lead to Knight Lautrec of Carim.
In the area with the Giant Stone Knights, a large tree blocks a path. It will move by turning, facing your direction, but it cannot harm you. Kill the tree to open the path. Another tree will be found further down this path.
There are two breakable walls that can be found at the end of boulder-paths.
The first is behind the resting Serpent Soldier that leads to the imprisoned Big Hat Logan. It cannot be broken with regular attacks. It can be broken by rotating the boulder-trap switch to smash it open with a boulder, or it can be broken by getting the Serpent Soldier to attack the wall.
The second is behind the boulder pit near the Mimic chest. After several boulders fall into pit, it will fill up and allow the next boulder to smash through the wall. This leads to the Covetous Gold Serpent Ring.
On the roof of the Annex, a breakable set of wooden boards leads to Velka's Rapier.
In front of the developer's message saying "Treasure" is a breakable stone wall leading to the Darkmoon Seance Ring.