Case 53s, known to Cauldron as Deviants or Subjects, were amnesiac parahumans, usually with severe physical alterations, branded with a "C". They were so-called because their origin was the 53rd anomalous case investigated by the PRT.
Case 53s were found in out of the way locations in urban areas across North America and around the world. They all had retrograde amnesia, with their first memories being waking up in a strange city.
All Case 53s were marked with the same brand, tattoo, or mark; resembling variously a Greek Omega, a strange latin 'U', or even a stylized 'C'.
Case 53s were monstrous in appearance, though exceptions like Shamrock exist. As the years passed Case 53's were increasingly less "monstrous" as time went on. Faultline estimated that four out of five monstrous parahumans were Case 53s.
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Case 53s were the results of experiments by Cauldron, successful in creating parahumans who did not immediately die from their physical mutations. They had their memories erased by the Slug to prevent them from sharing Cauldron's secrets, and Cauldron's symbol (a "C" drawn to resemble a stylized cauldron) branded or tattooed onto them with an ID number hidden in the right ‘arm' of the tattoo that each subject received; disguised as small spots where the tattoo hadn't taken.
Case 53s were usually taken from situations where they were about to die, their injuries healed or compensated for by their powers. Cauldron would experiment on these "subjects" by giving them doses of formula to test the results. Only two in five survived. Those who were physically transformed, becoming monsters, were known as "deviations" or "deviant cases" regardless of whether they were test subjects or customers. This possibility could be reduced using the balance formula, but it also reduced the strength of the resulting powers.
Only about two-thirds of Case 53s are monstrous. Some Case 53s were put towards the "Nemesis program", released with their memories erased and key weaknesses inserted into their psyches. Having noticed that Scion reacted aversively to some Cauldron capes, Cauldron deliberately spread Case 53s in areas where they were active, in order to distract him. Less than a hundred "deviants" were released onto Earth Bet. The vast majority were kept as prisoners in second basement level of the Cauldron Compound. Some of them became brainwashed Cauldron operatives.
↑ 1.01.1 What Alexandria had termed Case fifty-threes, after the fifty third file in a series of unresolved, difficult-to-explain parahuman events, one of the only ones to truly develop in their records. The Doctor had termed them deviants. - Excerpt from Interlude 28
↑There were countless messages pouring in, each something that had been flagged as a point of interest for Dragon. Every message on Parahumans Online that contained the word Scion or the phrase ‘end of the world’, every reference to a class-S threat, even crime scene reports that raised questions. He pored through them. Some kid inquiring about an Endbringer cult. A case fifty-three appearance in Ireland, with deaths. A woman claiming she could control Scion. A tinker claiming he had a bomb that could start a new ice age. - Excerpt from Interlude 26.x
↑IIRC, at one point they examine the tattoo under a microscope to read the number that's hidden within. It's a way for Cauldron to keep track of the Deviants when the deviants might change shape or be otherwise difficult to keep records on. If Deviants show up without the brand, then it suggests a scenario that might need Cauldron attention (ie. an Echidna like situation, stolen vials, other successful/unsuccessful experimentation around the world, etc.) - Comment by Wildbow on Spacebattles
↑She was at the cloud-level when the door opened. She stepped into the brightly lit corridors of Cauldron’s testing laboratory and strode down to the cells.
Thirty cells, filled with subjects. Thirty-one now. The cells didn’t appear to have doors, but the individuals within were all too aware of the dangers of stepping beyond the perimeters of their cells, or of trying to harass Alexandria as she strode by.
Only two-thirds of them were monstrous, affected by the formulas. Others would go free with alterations to their memories. Some would have fatal weaknesses inserted into their psyches, reason to hesitate at a crucial moment against a certain foe. - Excerpt from 15.z (Donation Interlude 3; Alexandria)
↑Entering one cell, she brushed the hair from the young man’s face once more, then propped him up while she administered the sample the Doctor had left for her.
She stepped back while he convulsed, his wounds filling in, his breathing growing steady enough for him to scream.
↑It would have gone somewhere if I'd gone more into depth with Faultline's Crew, giving them more of a spotlight. It was only an incentive to give people to get them formulas and push them to a higher standing when Cauldron needed people with good reputation in key areas (ie. to promote through the ranks of the PRT without too much interference). If you signed up, they'd release a case 53 or Cauldron subject with a bit of brainwashing, an implanted trigger that would cause them to lose. A keyword or other trick so the person who bought into the program could get easy wins against a potentially powerful enemy. - Comment by Wildbow on Spacebattles
↑As a distinct, noticeable trend, Cauldron capes are less inclined to go off the deep end. They wouldn't be more prone to reinforcing behavior or subtly using powers to generate conflict. At most, they have residual patterns/programming, that aren't tailored to their host. (ie. go subtle, let them think they have more control than they do, then in a moment of high emotion, activate power). A simple formula that's going to generate high conflict a great deal of the time. - Comment by Wildbow on Did Canary's shard...? on Reddit