There is a classification of entity that doesn't seem to be understood, if at all believed in. Some would say they're a hoax, or the result of another sort of entity. Among those who research entities, this type is often regarded as conspiratorial, placed into the same sort of category as Tree People instead of properly researched with care. Given their incredible ability to hide from plain sight and any sort of suspicion, I would not blame you for believing the same. However, I strongly believe that these creatures- of which I have dubbed "Outlanders," are not only among us, but are actively watching us struggle against the consequences of their very existence.
It is not entirely understood what an "Outlander" is, or what they do, as information on them is scarce and unreliable. They are terrifyingly skilled at keeping themselves out of anyone's sight, and can blend into our lives as effortlessly as a shark swimming through the sea. I believe their numbers are very low.
It is not known where they come from. What I can be sure of, however, is the fact that however they choose to live, it requires the violent- and lethal- assimilation of a vessel. The mind and body of a human is drowned underneath the overpowering existence of an "Outlander," and within due time, their existence is rendered null and void as the entity takes over their life in full. They will pretend- with great elegance- to be those close to you, and it is your job not to believe them.
Signs of Assimilation:
The specific abilities and traits of "Outlanders" themselves are shrouded in mystery, and many mistakes may be made differentiating them from other entities. However, there are ways to potentially detect if one of your peers has been assimilated. It is important to note that you should not check any of these off if these traits were true for the human even before assimilation.
A long-term symptom of assimilation is the changing of one's appearance. This may go hand-in-hand with disappearances. It seems that most "Outlanders" are capable of changing the body of their original victim to its will, making them a type of shapeshifter. It is unknown how much of their body they can change, or how long they need to grow accustomed to their vessel before they can achieve this.
This species is highly secretive, and your odds of knowing that an "Outlander" is indeed within sight is very low. It is very unlikely that you will survive if you have been picked out by one as a victim of assimilation- the best you can do is pray you are never in such a situation to begin with.