The Faceless are ghostly spirits. They appear exactly as they did at a certain point in life, often a point during which the majority of their memories reside. They may even easily interact with people even when they are very weak by manipulating the person's thoughts into believing they are physically there and can be touched, seen, heard, and smelled exactly as they were in life. They do not do this on purpose. As they are essentially powerful memories, the electrical impulses in the brains of those effected can easily assimilate the memories which make up the Faceless so completely that they "remember" the Faceless as though those memories are events occurring in the present.
The Faceless have one major function, which is to make sure they are not forgotten. New memories of them cannot be formed (as all new events in which they are involved are simply working around already-existing memories), and as the living people who remember them forget about them or die off, the entity's face gradually becomes more and more obscured, as though with a thin membrane of skin. When the face is entirely gone, the entity begins to lose all power and eventually is relegated to standing silently in corners watching events transpire around them. People may see them out of the corner of their eyes, but will most likely not acknowledge them or interact with them ever again.
This outcome seems apparent to the entity, as the closer this comes to happening, the more they will do to ensure it does not. There really is no set method they use, but they often become very inventive in trying to find ways to work around the limitation of not being able to directly form new memories. For example, a Faceless may not be able to have a unique conversation with you to tell you about their predicament and why you should remember them, but if they frequently walked in the garden while they were alive and happen to notice a prowler in the garden, they may walk past him to bring him to you, and then stand there watching you while you are robbed and assaulted so you will remember their involvement in that event, even though they weren't "really" there.
None of this returns their faces to them, but simply slow the process. The Faceless will stop at nothing to stave off their eventual manifest oblivion, and are only limited in their actions to the things someone still alive somewhere in the world remembers them having done.
What this means is that, if in life that person beat someone to death, they would then be able to ALWAYS beat ANYONE to death who may be unfortunate enough to encounter them. They are still in control of their thoughts and actions outside the limitation of only being able to interact with you through things they are remembered as having done, which means they may choose only to beat you HALF to death, if that suited their needs. However, every step of the way up until that point would play out exactly as it did the original time.
The only way to effectively neutralize them is to wipe out all memory of them. This does not destroy them, but simply removes their agency to perform actions in the physical world. They may also be banished or sent to other planes of existence given suitable occult knowledge.