Per 
Special Containment Procedures: SCP-5119-A and SCP-5119-B are to be kept in a completely soundproof 10x10x10 metre cube, and are to be given food by D-class personnel with musical backgrounds during the "neutral" phase of SCP-5119-A's cycle.
D-class are to clean the room during the "dormant" phase. Complete silence is recommended.
Addendum 1: Following incident 5119-1, two AA batteries are to be provided alongside the food once a week.
Addendum 2: All interviews and experiments are to be approved and led by Dr. Melanda Cullings.
Description: SCP-5119-A is a 1.8m tall mottled humanoid wearing a tailcoat, black tap-dancing shoes, pinstripe dress pants and a simple white shirt.1 It's fingers end in sharp claws, and the creature's face vaguely resembles the skull of Canis lupus familiaris, the domestic dog. Dr. Peterson noted resemblance to a St. Bernard skull specifically.
SCP-5119-B is a small cassette-tape player that never seems to play the same tune twice. No other anomalous properties have been observed. SCP-5119-B appears to be connected to SCP-5119-A on a fundamental level, able to translate SCP-5119-A's native internal music into conventional audio.
The sound it produces holds no apparent anomalous properties.
SCP-5119-A and SCP-5119-B were first located in Lusk, Wyoming when a local resident called the police about loud music and screams next door.
After the police that had arrived failed to leave, Mobile Task Force Epsilon-6 ("Village Idiots") were called in to contain SCP-5119. They found the bodies of three residents, one with their skull caved in and two with broken ribs and punctured hearts. Both of the police officers had their throats crushed. Several shots appear to have been fired, but no wounds were observed on SCP-5119-A.
Class-A amnestics were administered to all living civilians involved.
SCP-5119-A has three phases: active, neutral, and dormant. During the active phase, the creature dances in time to the song played by SCP-5119-B, which can last anywhere between 1 and 7 minutes. The time between songs can be between 1 and 4 minutes, during which it acts like it does during it's neutral phase.
Both the tempo and the key can vary drastically, ranging between 23 and 316bpm for tempo. All possible keys have been used.
"I could list the time signatures, but there's not much point. Every time signature throughout history has been recorded and then some." -Peterson
SCP-5119-A will attempt to keep everything within range of sight and hearing in time. It will approach things that it considers "out of time", (such as irregular speech, movement, and sounds in general,) with it's pace dictated by how "out of time" the object it approaches is.
Once SCP-5119-A is close enough, they will attempt to destroy whatever/whomever is out of time. This phase lasts anywhere between 9 and 14 hours.
During the neutral phase, SCP-5119-A will stay at 100bpm and 4/4 time, no matter the tempo or key of the song previous. During this phase, SCP-5119-A seems much more lenient about things being "out of time", as it's approach is much slower as compared to it's active phase. This phase lasts anywhere between 4 and 9 hours.
During the dormant phase, SCP-5119-A will enter a sleep-like state. This phase always lasts exactly 6 hours. From close examination, it has been discovered that both it's breath and it's heartbeat keep with the 100bpm tempo. It is assumed that the same is true for it's active phase.
Incident 5119-1:
At 14:54, five days after SCP-5119-A was recovered, SCP-5119-B cut out mid-song. SCP-5119-A immediately stopped dancing. Two D-class personnel were sent in to check on SCP-5119-B. During their checkup, SCP-5119-A approached and killed them both.
"It was at this point we learned that SCP-5119-A isn't dancing to SCP-5119-B's song. SCP-5119-B is merely transcribing SCP-5119-A's internal music into an audible form." -Peterson
After a few minutes passed, SCP-5119-A opened up the player and took out two AA batteries. It placed them upright and waited.
When it was time to feed it, two AA batteries were added to the food tray. SCP-5119-A inserted them into SCP-5119-B, and left the dead batteries on the tray.
"SCP-5119-A clearly understands the concept of batteries. Is this because of SCP-5119-B or some other reason?" -Peterson
Further experimentation:
Dr. Jacob Peterson attempts to interview SCP-5119-A remotely during the neutral phase. "SCP-5119-A, can you understand me?" SCP-5119-A approaches the speaker. "SCP-5119-A, I have some questions regarding your origin. Can you-" SCP-5119-A suddenly leaps up the 10m wall and smashes the speaker.
It was repaired during the next dormant phase.
"So far SCP-5119-A has shown no signs of limitation when it comes to correcting rhythm. As long as it can hear or see it, it's able to destroy it." -Peterson
Dr. Peterson orders a music player to be placed in the room with the food. SCP-5119-A ignores the device and continues as normal. During the active phase the following day, the player is activated remotely. SCP-5119-A immediately attempts to destroy it, crushing it in three rythmic stomps.
"That went about as expected." -Peterson
Dr. Peterson orders a D-class who knows how to dance to enter the room during it's active phase. The D-class lasts a total of 56 seconds before SCP-5119-A snaps his neck.
"Not knowing what music SCP-5119-B will play is a bit of an issue when figuring out how they both work." -Peterson
Dr. Peterson orders no new batteries to be delivered. SCP-5119-B runs out of power near the beginning of the next active phase. Close examination reveals that SCP-5119-A is still bouncing along to a tune no-one else can hear.
"If only I knew the origin of SCP-5119-B. That would make this much easier." -Peterson
One Dr. Melanda Cullings has been assigned to SCP-5119 due to her musical background.
Cullings enters the room during the neutral phase, accompanied by two D-class personnel who have experience with feeding him.
She speaks in time with SCP-5119-A rhythmic nodding. Comma's rest for one beat, periods for two. The D-class stay by the door.
Log:
"S C P 5 1 1 9 A, can you un der stand what I am say ing?"
SCP-5119-A nods heavily.
"Good. Do you know where you came from?"
Pause for three beats. SCP-5119-A shakes it's head.
"Do you have a name for your self?"
Heavy nod.
"Can you tell me?"
Shake.
"Can you speak at all?"
Two shakes. SCP-5119-A takes a half step closer.
"I think we will take our leave now."
Dr. Cullings turns and walks out, keeping in time. The D-class quickly follow and shut the door.
Dr. Cullings requests permission to enter during the active phase.
Permission is denied.
Dr. Peterson requests permission to remove SCP-5119-B from the containment chamber.
Permission is denied.
Dr. Cullings is found in the observation booth dancing along with SCP-5119-A during the active phase.
Permission to enter is granted.
"If that performance is any indication of her skill, I'd give her good odds of surviving this." -Peterson
Log recorded by Dr. Peterson:
Dr. Cullings dances into the containment chamber and shuts the door behind her.
SCP-5119-A showed no sign of surprise, but Cullings later claimed that it seemed happy.
167 seconds into the song, Dr. Cullings began to sing out questions.
"Do you know who you are?" "Do you know what you are?" "Why were you in Lusk?"
SCP-5119-A responded by singing back. It's lyrics don't match any language on record, though the cadence is vaguely eastern european.
The song ends at 228 seconds. Dr. Cullings bows, and SCP-5119-A bows back.
She exits the chamber.
As of ████/██/██, Dr. M Cullings has been put in charge of SCP-5119-A.
Dr. J Peterson has been relocated.
Dr. Cullings requests a toy piano. "I may be able to communicate more effectively through music."
Permission is granted.
She enters the chamber during the neutral phase with the toy pianos and a notepad.
Log:
Dr. Cullings places the piano in front of SCP-5119-A and sits a short distance away.
"S C P 5 1 1 9 A, do you know the notes?"
A nod. SCP-5119-A plays a C-major scale.
"Can you make words with the notes?"
Another nod. The creature pauses for a moment, then plays F E E D.
"Are you hung gry?"
Nod.
"Food will be sent in short ly. An y oth er re quests?"
A pause. B E D.
"I will see what I can do. Good bye."
Dr. Cullings stands up and exits the chamber, leaving the piano. SCP-5119-A waves goodbye.
"So we now have a method of communication, frustrating as being restricted to seven letters is. I have a good feeling about this." -Cullings.
Dr. Cullings requests that a bed be brought into the containment chamber.
Four D-class transport a king-sized single bed into the room during the dormant phase.
TRANSCRIPT OF AUDIO LOG
"This isn't a test log. This technically isn't even science. This is just a recording of my suspicions and hypotheses that I have formed. Firstly, it is my belief that SCP-5119 is just as intelligent as any human. They have shown an understanding of human speech, along with things such as batteries and piano keys. I suspect that they were raised as a human, or perhaps they were a human at some point? Anyway, I hypothesize that the reason SCP-5119 destroys things that don't match it's rhythm is because of a form of anomalous OCD. Is this possible? I don't know. I'll have to ask someone who knows about that sort of thing. End recording."
