Simulacra

"Able to pass as men, they could infiltrate all areas of society. Given time, they could replace planetary governors, work their way into the Imperial Guard and Navy, and even hide within the Inquisition. It is as I have said before: all are suspect, none are to be trusted, and even the most benign of actions should be investigated. I hope the scope of this threat is only within my mind, for if not..."
–From the journals of Inquisitor Felroth Gelt 100.801.M41
There is a foul and frightening breed of xenos hidden away
in the crowded warrens of hive cities all over Calixis Sector.
Known only by rumor for many centuries, recently one of
these beings was dissected after being killed. The autopsy
revealed a lifeform capable of changing its shape. Nominally
humanoid in appearance, with an almost featureless face, no
body hair, and skin so pale as to be translucent, this creature’s
internal physiology allows it to manipulate its basic form in
order to imitate others. With a physiology that renders it
both flexible and durable, this new form of xenos seems to
be as strong as any human, but somewhat more agile. They
have no natural weaponry, instead using human–made arms
and armor.
By all accounts, these beings, known as Simulacra, are
fully sentient. Able to mimic others well enough so to live
unsuspected among humans for extended lengths of time,
their true motives are unknown, but no one in the Ordo Xenos feels it’s benign. When one considers a Simulacra’s
feeding habits, it’s easy to see why.
In order to imitate someone, a Simulacrum needs to
consume that person’s brain. Through some unknown means,
the Simulacrum ingests the brain whole and more or less
intact. This consumption allows the Simulacrum to assume
that person’s appearance, speak with his voice, and recall his
memories. It also allows the Simulacrum to use its victim’s
skills. However, as the brain is slowly digested (a process
that takes roughly four weeks), the Simulacrum’s ability to
assume the victim’s form or use skills degrades. Once the
brain is fully digested, the Simulacrum can no longer assume
that form or use those skills.
As a Simulacrum eventually loses whatever shape and
skills it gained from a devoured brain, it usually tries to hunt
down a new victim every week, using any skills gained from
any brains it is currently digesting. Simulacrum tend to go
after people who it perceives as having useful skills or status.
It is suspected this is so the Simulacrum can slowly move
its way upwards in a hive, both physically and socially. One
can only imagine what a Simulacrum would do if it gained
access to the nobility of a hive. Some Inquisitors argue the
creature would be able to hide away in a spire, living off
members of the lower classes brought to it by unknowing
servants. Others suggest it would bring others of its race
with it, intending to eliminate the noble house from within,
before starting on the hive itself.
The existence of Simulacra seems confined to Calixis
Sector, specifically the Malfian sub-sector, with its close
concentration of hive worlds. Currently, the Ordo Xenos is
trying to track down the homeworld of the Simulacra (if
there even is one) in order to destroy it. Curiously, there have
been reports of beings like the Simulacra from worlds outside
of the Malfian sub-sector. It’s unknown whether these are the
same type of creature, or if an entirely different lifeform is
being described.
Instances of smuggling relating to the Simulacra are not unheard of. Inquisitor Joshua Starkh, at the time not even an Acolyte, helped break up a smuggling ring on the world of Siren's Den, in which the Simulacra were being used a playthings for the corrupt and the decadent.