The living wall is one of the more powerful and mysterious creatures listed in the AD&D Monster Manual (AD&D 2nd Edition version).
The wall will appear to be a normal wall of stone or brick, although creatures with true seeing abilities can see what it truly is. It is actually a wall of rotting human flesh and bones, with faces, bones and limbs jutting out in random places. This wall strongly radiates both magic and evil, but infravision cannot detect it. A constant low moan emits from these walls, even the deaf and those effected by magical silence can hear it.
A living wall is made out of the bodies of creatures who have died within 100 yards of it. Everytime the living wall kills someone they are assimilated, which adds to the wall's power. The assimilated creatures retain their intelligence and powers, but become utterly evil and seek to kill all creatures that approach the wall. Any absorbed wizards or clerics will add their spell powers to the wall. The same for warriors, the wall will retain their weapons within, and produce them as needed (to be wielded by one of the arms jutting from the wall). Ranged weapons are rare, as the wall has no way to regenerate the ammunition required to fire them.
The wall expells most nonmagical equipment from assimilated creatures. This leaves a nice pile of loot at the base of most of these walls, which only serves to attract even more people to be assimilated. When someone comes close enough to the wall, they will be grabbed by the hands, which will attempt to pull them into the wall. No one sees the walls true nature until they are touched by it. When attacked the wall will strike back one time for each time it is hit, for each person it has assimilated. (If a wall is made of 14 people, it will attack 14 times in return for each time it is hit). The type and skill of the walls attacks are dependent on the assimilated creatures.
The living wall does not reproduce, but they do grow and grow as it assimilates more and more creatures. The Monster Manual says that the wall is limited to a single section of wall, and thus will not turn corners. But the only module I ever saw that used the wall contradicted this. So I will have to agree with the module and say that the wall can grow to engulf an entire structure.
A living wall is either created by an evil Archwizard, or is created naturally in especially evil lands (such as Ravenloft). Above all else the wall seeks to destroy it's creator. Only then will it revert to being a normal wall. The walls are best avoided when discovered, as any wall more than a few years old will be almost impossible to destroy, and will gain power with every attempt.