For the full theory, follow the links down the chain (REPLY HERE)
It's got a lot of fluff that kinda derails things, so i decided to smash it down into its most key points
=========== Mikepurg ===========
Also known as "Mikehell", it's a theory which states that the protagonist of UCN is Michael Afton, not William. This is the entire basis of Cassiduo, so the following will focus on that first:
= UCN =
There are plenty of lines saying how the guard is human scum & needs to be punished for eternity, but... there are no lines directly naming him as the killer of more than one person (TOYSNHK), only indirect implications.
Also the "Miiiike!" OMC easter egg has no consonants pronounced, except a K at the end of every ""word"".
Let's face it, Willhell is easy. Like, stupidly easy. So why is there ambiguity in the first place, if the answer wasn't the one that's buried in the details?
The entire Bear of Revenge has multiple parallels to CC being the (golden) bear, and Mike being the Fox:
The Bear has a birthday, turns golden when the sun sets, and knows the Fox (and his victim) very personally
The Fox has a favorite show, defends his enclosure for 5 nights, and gives juvenile punishments to the Bear
Also Mangle with a headband is in the Logbook next to Mike's wish to "take a self-defense class", like say martial arts in a Dojo, perhaps earning a black belt in karate?
= Logbook =
The conversation in this book takes place between 3 characters and a 4th after the fact:
Faded text (Cassidy)
Altered text (Evan/CC)
Red pen (Michael)
Tally marks (Henry)
The only thing that could make Cassidy the MCI victim is a single gravestone, and is the only thing connecting Cassidy to William at all, but the gravestone is in red, and the Henry puzzle has you add up all the tallies & get that page's number, and Henry writes in red.
Cassidy only ever talks about CC, which is a point i'm pretty sure Willhell takes completely for granted, because CC's only connection to William is bloodline & the SL secret room (maybe fredbear plush?), and no one should even know about that, let alone CC himself.
== The Major Problem(s) ==
Cassidy has to know about Fredbear plush & everything in CC's life, but also tell their name to Michael so he can finish it, which doesn't seem very likely for a character with no solid backstory.
Most importantly, if Cassidy isn't the MCI victim, who the hell else knows CC down to his feelings, hobbies & talking plushies?!?
=========== Enter Cassiduo ===========
A theory which states that the Crying Child is both Evan and Cassidy at the same time. That's how they know CC so well, how Mike knows their name, how they know about Fredbear plush, and why they especially want Michael to suffer for eternity.
= Books =
Disclaimer: Scott says the frights series "fills in blanks of the past", which i think means it at LEAST parallels game lore in some useful ways, like how Step Closer's Pete connects Michael to Foxybro through the logbook.
There are two very important kids:
Jake: Parallels Evan. He talks to a stuffed friend, fabricates a better version of himself, dies from a fatal condition, then becomes a doll who was voiced by a man named Evan who has a brother named Michael.
Andrew: Parallels Cassidy. He splits himself to control many things at once, takes on their personalities, is very bitter, and torments an unrecognizably burnt man for his misdeeds.
Both kids possess the Stitchwraith, an animatronic in which Jake is the main pilot who can see & move, whereas Andrew's more in the backseat (as he often is), which tells us Cassidy's the personality, not the person.
As well, they sort of cross-parallel each other, like Andrew being unable to see & having hair similar to CC's, while Jake is the one trying to communicate with the other spirit, and became the toy that parallels Fredbear plush (point for plush voice being Cassidy instead of William)
= Nightmarionne =
In the Freddy plush teaser for the halloween teaser, brightening the image reveals Nightmarionne as a shadow coming from the now-golden Freddy plush, implying that's what this character was at least meant to be
Even further, a FNAF 6 teaser has the same exact Freddy plush black bg, but this brightened Freddy has strings attached, which are all segmented the same way, resembling very thin Nightmarionne tentacles, meaning Golden Freddy is no longer in control. Makes you wonder why he twitches in UCN...
(im aware the image has other meanings)
=========== Charlotte's death ===========
A sorta separate add-on to the theory that states that CC and Mike were both witnesses to the aftermath of Charlotte's murder
Nightmarionne and Phantom Puppet both have eyeballs and tear streaks in their design, but we see in Security Puppet's minigame that the Puppet loses its eyes before gaining its tear streaks, which likely means this minigame doesn't show the whole story...
Here's the scenario: Mike & CC sneak out to visit "JR's" aka Fredbear's, Mike pranks Charlotte by trapping the puppet & locking her out of the building. Purple guy kills Charlotte, tires go brr, so they check on her only to find her dead. CC's mind created Nightmarionne, and Mike's created Phantom Puppet.
CC gets followed by a worker in the Help Wanted rabbit costume (no spring-locks in the rain!!!), and CC breaks the window to get INTO the house & lock his door, while Mike just walks home in severe shock.
William drives far away from the scene to dispose of evidence & get a well deserved drink, and Later that night, Midnight Motorist happens.
Henceforth, CC blames Mike for her death, and for creating Cassidy & their fearful reflection, Nightmarionne /)'o'(\
Whew, now where's my cookie?