Per 
Item #: SCP-4215
Object Class: Euclid
Special Containment Procedures: SCP-4215 must be kept in an underground, soundproof bunker. The door to the containment area must be metal and at least 20 inches thick. If SCP-4215 is, for some reason, contained in an overground base, the walls must also be 20 inches thick. This is to stop any sound getting out because it can produce sounds of almost triple what is safe for a human. When entering SCP-4215's containment area, all staff must wear military grade ear protection (this will not block out all noise it will just reduce it), a hazmat suit and thick boots.
BY NO MEANS TAKE OFF PROTECTION WHEN IN THE PRESENCE OF SCP-4215, BY DOING THAT YOU ARE PUTTING YOURSELF AT RISK OF DEATH.
SCP-4215 must always wear a mouth restraint to reduce the damage that could be done if its screams are heard. When SCP-4215 is being moved and transported it must be tranquilized until it's in its suitable conditions.
Description: SCP-4215 is a slender, agile creature with a dull grey skin it often blends in with the colour of the walls. Fortunately, its mouth restraint is a light blue colour so that staff can see it. SCP-4215 has a wide mouth with no lips. Around the mouth area is slightly tinted green, most likely due to the acid, fluorine monohydrochloride or FMHC, produced in its mouth. It has no teeth and its gums are also tinted green, smell like rotting meat and are covered in an oily, acid substance, FMHC. We managed to collect some when it was sedated and is being tested in the lab. If you are not wearing the correct clothing and FMHC lands on you then within 10 seconds your skin will start to boil and your skin goes a grey colour; your teeth will fall out and you will screech for around 20 to 30 mins. After about 40 minutes you will start to twitch and spasm. And after about an hour you will be dead.
The reason anyone that comes close to SCP-4215 must wear ear protection is that it screams at 240 decibels and at a very high pitch if you hear this noise with no protection your brain will implode due to the immense noise. Another reason is that it often uses subliminal noises to control others for its benefit. When SCP-4215 screams FMHC is sprayed everywhere. Luckily FMHC doesn't burn through clothing. When with doctors SCP-4215 is not hostile and tries its best to cooperate however when with armed soldiers SCP-4215 gets angry and becomes hostile. The subject does show some signs of intelligence however does not know how to talk but does try to communicate using its hands and a few short hums and clicks.
Addendum: It seems that in some very rare cases human subjects that have been in contact with FMHC only go through the first 4 stages, then after that the human subject starts to become more slender and their lips start to thin, so much that the lips have 'disappeared'. Eventually, the subject becomes SCP-4215. We still don't know if the human subject has its memory of being a human and why it is where it is.
Addendum 2: There is no recorded natural death of an SCP-4215, however, this does not mean that they are immortal but that they live for a very long time.
