Okay so here's the wall of text about the Sentinel tower lore. Maybe everything I will write will prove to be wrong but I tried to reason according to every hints Teary put in the game while not overthinking things
To keep things structured I will first give analysis on the events we saw before giving conclusions or theories :
Let's start with the "Rampage". The rampage is the name Alex uses to qualify what Julia ( the bloody girl ) did when the MC fell under the tower for the first time. She basically killed and ate every living entity down there except Alex, the MC and Ki ( the lost silent boy ) if you chose to save him. It's confirmed by Alex that they are the last living things in this section. ( except when you inspect some cells in the "kitchen", it says there are people in it so it might be an inconsistency ) After that, Alex cures Julia's current state. When interacting with the latter, we can see that she's... normal. She feels guilty and is most likely traumatized about killing all these people. She probably has a hard time remembering the events. Before getting to the interesting part of her explaining how it happened, it's interesting to note a dialogue from Alex, when you question her about her role in this mess.
She answers that she has to "fix" the "rampages" whenever they get out of hands. It's important to note that she says fix, and not PREVENT. As if it was... normal for the rampages to happen.
That's where the explanation of Julia is even more interesting and understandable. Julia explains that soon after her start at the college she was brought here under the tower and that the last place she saw was Allan's office. Allan apologized for doing this and then, while Julia was blindfolded there was Miss Cho ( a "teacher" who is not often saw at school ) who started repeating things in her ears. She spoke about various things : "let go of the control", "eat",You have to be "watered", You have to "grow", and the most interesting thing she said is that a catalyst was near, and that she ( Julia ) had to be ready to serve Niht. ( I'll explain later but this Catalyst is Miki at 99% )
So what can we assume knowing all of this ?
The first thing that comes to mind is the fact Julia was almost possessed while doing her rampage. It wasn't her wish to kill everyone. Yet she did, and while we could say it was an accident we have enough elements to guess it's probably not the case. Miss Cho spoke about how she needed to EAT, to GROW, to be WATERED. She treats her like a kind of plant. But... Instead of watering her with actual water, we can imagine she had to water her with blood. Hence why she pushed her to consume all of the peoples down here, and that also explains why Allan was apologizing. He probably knew what was going to happen but as the character said himself he has his "hands tied."
Now to go more in depth, we need to know what is a "Nihtling" in this story. Dominic's diary informs us that Niht is a primordial God, on of the most ancient of the world. His powers is manifested by blood and nails. We have no idea what this entity is or want but he doesn't seem friendly at all. The Nihtlings, who are some kind of progeny or "childs" of him, also inherits more or less of his magic. Walter ( the student who tried to kill the MC before he was resurrected again by the succubus possessing him ) is seen using bloody nails while trying to kill MC.
The principal, is litteraly nailed down to her chair. There is also a cult room under the tower where people prayed him. It's easy to see the Nihtlings of the school who also seem responsible for what happened at the Sentinel tower are some kind of a cult/fanatics.
There are these mysterious pearls also. Teary left less hints about that. The big informations we have about this is that Walter, IS a pearl. It's written in his character info. ( you can also notice he's munching his cravat in his splash art. He's HUNGRY, he has to EAT ) So we do know pearls can become Nihtlings. We also know they can become "failed pearls", a pile of meat searching to eat... Human bones, and even walls. We have no clue about where those pearls are from but they can produce Nihtlings, and seem to grow by drinking blood and eating themselves, in fact the same words are used for the growth of pearls as for the already adults Nithlings, such as Julia. There is also mention of deprecated pearls.
With all of this in mind, the blood, the nails, the cult, the pearls... I think Teary went with the thematic of the sacrifice with the Nihtlings. And while succubuses need to suck people's energy to grow stronger, Nihtlings could be needing to EAT people, or maybe suck the blood to grow. That could explain the stuff Cho was saying "grow, you need watering, eat etc..." and this is also words which come back regularly in the tower, on the interactable such as the notes in the kitchen, the .txt on the PCs etc... the process of "watering" plants ( who are in fact Nihtlings probably ) and the fact the foodstock in the kitchen was full of blood and is also the spot where we found Ki shows that some flesh was on the plate of the canteen. ( Unrelated, it's also good to know that Ki is a human in the character info. He's the only human related to the tower, he's called "deprecated" by the nihtlinngs. Are the deprecated former Nihtlings who lost their powers ? )
All of this shit seems to have been planned, as if Cho wanted Julia to actually eat everyone ( not sure about the staff tho, maybe her rampage went too far ) to grow.
When you check characters status you can now see julia is a "Night Walker". Beatrice, and the dude with the glasses are also Night Walkers. Trent is probably one too. There are chances that "Night walkers" are an "elite" type of Nihtlings and that the purpose of the sections under the tower is to create those ones. Not sure why Trent Beatrice and the others aren't traumatized like Julia tho.
To conclude this essay we can link all of this stuff with the event of Miki's disappeareance. A female Nithling came to the tower to meet Miki. She smelled her and noticed she was a succubus, then asked if she was Noo ( Noo probably played a part in the Nithling schemes ) to conclude Miki was indeed what she's been searching for. We must keep in mind that Miki was not summoned in the tower randomly. Someone ( probably a nithling ) used the ritual of Dominic to lock her in the tower, so since the beggining of the game she was a captive. They already knew Miki was a... Catalyst. So the woman called her superiors and they most likely brought Miki to the school to achieve something. How do we know Miki is a Catalyst ? First, Cho said to Julia that a Catalyst was near. Then we know Miki is an hybrid Succubus/nithling. And well, if you come to Sentinel tower after Miki has vanished, you can interact with a lot of little spirits where she was staying who are saying very interesting stuff. One of them straight up says somehting like "A perfect catalyst. Niht will be pleased." and another says "If she's your friend, take her away before they do". That leaves no room for doubts. In other maps others spirits say stuff like "Niht is back, i'm relieved" or they don't understand they're dead. I won't extend too much on that but there are chances that these spirits are mainly the people Julia killed recently and they know what's going on.
Here comes the conclusion : Considering all the elements we have, it comes naturally to mind that the Nihtlings just want... To "resurrect" or summon Niht himself. Judging by the dialogues, he is already there but maybe he needs a body or a ritual to come back permanently. Here comes the principal. How could she not play a role in this ? She's the highest member we know in their organisation. She's nailed down to her chair, deblatering enigmatic stuff except one sentence : "How long will they take ?" We can assume she's waiting for her underlings, the Nithlings of the school to achieve the ritual or whatever they planned. The fact that she's waiting, nailed down like this makes me think her body could host Niht. They needed Miki and probably other stuff done ( maybe Nithlings sacrifices ? ) and where the story stop it feels like they are almost done.
Phew writing so much about a porn game make you question your life. If someone has the willpower to read this, don't hesitate to tell me what you think about it already !