Warning: Spoilers Off applies to these pages. Proceed at your own risk.
The Elite Mook from the second chapter, especially given that it killed Gwen, Sally and Sylvia and mortally injured Undine, as well as the simple fact that, oh yeah, monsters swarm the city every single night and if not for the protective field, it would have already been destroyed, and nobody is sure whether or not other cities still exist.
It's practically stated that Undine only survived the Elite Mook attack described above by using her control over water to manually pump her blood around her body.
Imagine after a big argument, you decide to leave your friends to their own devices for one night. You go to check on them and then.... three are dead, and the the last one is dying. One practically embedded in the ground.
On the funeral of three of Team Alchemical, Undine is shown surrounded by the bloody, smiling and dead Gwen, Sally and Sylvia. Gwen and Sylvia even roughly stand where Undine's probable parents were before...
When Undine revisits the site of the tragedy for answers, she gets flashbacks to the last moments of her three friends.
The mysterious creature that looks like a girl who has been spotted several times already and was present at Undine's healing also has something chilling to say: "Poor little girl... you wouldn't be so confused if you hadn't forgotten."
Said creature really really hates Undine for whatever reason.
"Did you think you would find answers here... or did you just want another chance to die?"
The interstitial between chapters 4 and 5 states that with enough time and effort, a monster or even a Magical Girl can break the weaker barrier between the houses and the streets. And we already suspect a magical girl is working with monsters... the mysterious narrator even looks at the audience as she says, "But you don't have to worry about that just yet."
The interstitial between chapters 3 and 4 suggests that the events of Chapter 2 have only taken place four days prior, which suggests that Undine was checked for injuries and promptly left to her own devices. Kinda no real shock that she ended up snapping at Tessa and the reporter.
HP gets a phone call from none other than The Purple One, who even knows HP's full real name and says if she doesn't take care, HP will end up like her mother.
Heartful Punch explains exactly how her mother died, just a few months after giving birth to her. One night, her mother's former teammates, Starlight Spear and Sunlight Spear, see an emergency flare. Starlight Spear hurries ahead on the off-chance there is still someone she can help. Slowly, she realizes she is in the vicinity of Mitsuki's home. She stops at a house full of ice — Mitsuki's. Mitsuki's mother, still clutching the flare's empty cylinder, is dead on the doorstep. Starlight Spear cautiously goes in, calling for Mitsuki, unknowingly walking over the body of Mitsuki's father, frozen under the ice, then hears the inconsolable wailing of baby Kokoro. In the nursery, she sees Mitsuki, half-embedded in ice, slumped against a chest of drawers, turned sideways, and pushed in the way of the wardrobe where the crying is coming from. Mitsuki's dying act was making sure her daughter lived, and as Starlight Spear goes to her and touches her body. She first starts up a Rapid-Fire "No!", then starts crying as she gets a look at Mitsuki's face, eyes empty, hollow and crying blood.
Chapter 12's first page is about Tessa looking into a mirror panting, with a grey, stone-like texture spreading from her mark (a deep gash), her hair turning into brown and pink hair falling out. Burr...
Tessa is in such a bad headspace that panels are cracking when she is on screen. They only disappear when she is in the graveyard and come right back when she is trying to put up a facade for her parents.
So not only has Tessa's mental state been deteriorating, not only are people apparently forced to forget about the Girl in White, but according to Anemone, The Purple One has been lurking outside of Tessa's window at night. Sweet dreams.
She left a message for Tessa too: "I'll be here whenever you're ready." Ominous.
Cassidy's weapon/item is a giant pair of scissors. Her main power has her use it to duplicate herself... by cutting herself in half.
The interstitial reveals before getting the dream, Cassidy used to desperately wish to be a magical girl and fell asleep wishing for a chance. Whoever heard her wishes has a nasty sense of humour if they forced Cassidy to basically selfharm for the powers.
Cassidy is revealed as a Bitch in Sheep's Clothing when she accuses Undine of being responsible for her teammates dying and Tessa's depowering herself. She also claims that she's not the only one who feels that way, and that "everybody" agrees with her that she's been handling the loss of her friends far too well. Acting as though the rest of the Training Club members, along with others who only saw/heard the news about what happened, suspect that she's responsible for what happened.
Cassidy ends up alone, fighting against the cat-like monsters... and The Purple One appears to talk to her. Goops tries to trap Cassidy, which Cassidy gets out of by cloning herself, but then one of the cat monsters destroys both her clones at the same time. This causes her to die, with no trace of what happened left behind.
Aside from Cassidy going missing, a number of magical girls, including Harley, got injured during Chapter 15, to the point that Vedika said a few were hospitalized, and the healing based magical girls are exhausted themselves, pointing to the idea that what Anemone said was true: the situation's becoming unsustainable.
The interstitial for chapter 16 provides a Wham Episode that many fans have dreaded-Tessa is communicating with Goops.
ThisWham Shot in chapter 17: looking at the Outer Barrier causes the same automatic memory block as trying to remember the Dream or learning the identity of the statue.
The C.D.D. has started investigating Cassidy's disappearance. It makes one fearful about what will happen to the magical girl training club, especially since the C.D.D. knows about the fight between Cassidy and Undine, and the possibility that they may draw similar conclusions to Cassidy's thoughts about the demise of Team Alchemical: that Undine had something to do with Cassidy's disappearance. Mingxing at the least believes in their innocence but she's only one person.
Mingxing flashes back to the night Mitsuki died and she lost her arm. Her arm literally froze and broke off before she could defeat the monster, and it's implied that Kokoro nearly froze to death because of the monster's ice based powers.
Tessa is horrified at the idea that Team Alchemical could have been targeted, rather than it being a mere coincidence that they died the one night she sat out...and then the panels start fracturing again as she realizes she's still being targeted.
Cube made another exception to the 'no Alt Text while Goops is around' rule: during the flashback to Tessa's first time talking to Goops, the Alt Text says 'Mada dame yo'. Aside from the fact that it translates to 'not yet', "Mada dame yo" was the Leitmotif for Puella Magi Madoka Magica The Movie: Rebellion. Kinda ominious if you're at all familiar with Rebellion's story.
Goops declares that she did indeed attack Team Alchemical to get to Tessa. Tessa is understandably scared shitless.
You're the one that matters.
Tessa's thoughts as she sneaks downstairs. She blames herself for driving Rue away and for Undine being in danger, and above all, she feels tired of being useless.
Goops: You just want it all to be over, right?
We finally see what Goops's plan for Tessa was: to possess her.
The start of the next chapter not only ominously titled "???" but the first two pages can only be described as pure nightmare fuel, in what can best be described as Mind Rape in visual form.
What's worse, Goops's dialogue hinted that she's done this to Anemone before.
There's something eerie about the Woman in White's eyes.
Take a good close look at Tessa's eyes when she asks the Woman in White if she's the one that grants magical girls powers. There's static there. Even in The Dream the static keeps people from knowing too much.
When Tessa starts incorporating Goops's memories into her own, it's implied that the whole mess with the monsters was The Woman in White's fault in the first place. Hard to say what prospect is scarier: that Goops lied to fulfil her own agenda...or she didn't.
Tessa finally completes merging with Goops to become a magical girl again, who looks like a Dark Magical Girl take on her Alchemical Aether form.
"We'll just have to do it all ourselves. What else is new?" [gives a Slasher Smile]
The monster that attacks Kokoro in Chapter 19 does something that makes her invisible to Undine. Undine, predictably, is freaking out.
And then Kokoro starts disappearing entirely.
It's implied Anemone is quite literally a Barrier Maiden, or, in other words, the source of the inner barrier: her room, the objects, and even her hair and clothes are the same color as the inner barrier. Kinda scary to consider the only thing keeping monsters from attacking civilians is one unconscious girl.
And sure enough, Tessa Alter's first move is to try to break the barrier to get to Anemone. It's absolutely chilling to go from a monster firing a shot through the hole in Anemone's window to the inner barrier going away.
Fortunately, a later strip would confirm that Anemone is alive.
Vedika aka Mindful Eye, usually a non-combatant, draws a monster away from a healing center when the barrier is broken, attempting to fight it off with a flare gun. Though she manages to land a solid hit, the monster's claw grips her ankle, inadvertently activating her contact-telepathy powers and exposing Vedika to what lies within: a roiling mass of terrified souls, begging and screaming for someone to help them and let them inside.
And then Vedika's memory of that was erased by the memory block; when trying to recall it, she still feels the trauma of the vision, but doesn't know why.
The last thing Tessa Alter does in Chapter 20 is reshape herself so she looks like plain old Tessa again. Tessa's parents are about to harbor a huge threat to the city and they don't know it. There's also just something very Uncanny Valley seeing Goops's facial expressions on Tessa's (non-fused) face.
The more lighthearted Chapter 21 ends on an ominous note as Tessa Alter watches the interview on a tv screen.
"It's too bad you can't just let go. It makes everything so much easier. Eh, soon enough."
It's implied Anemone is so unsettled from her encounter with Tessa Alter that she's unable to concentrate on the inner barrier: instead of one barrier it looks like multiple smaller ones, and both Tessa Alter and a couple worm monsters easily broke through them.
Undine: Those little things already made a breach this big?
Undine later notices that even HP's little darts breach the barrier, pretty much cementing that the barrier is weak.
The monsters attempting to break through a door, and having broken the inner barrier (referred to by the author as "lil skitter guys") retreat elsewhere, and form parts of what looks like a blonde wig resembling the Woman in White's hair.
Undine's friends' eyes especially.