In Mortal Shell 2, the Marrow Keep features a unique illusory wall that leads to a special dungeon. The location of this illusory wall is quite hidden, so keep an eye out while exploring. Look for white markings on the hidden wall. Interacting with it will initially cause you to lose health, but on your second visit, you'll be able to enter the dungeon.
Where is the Illusory Wall in Mortal Shell 2's Marrow Keep?