There are many reasons why necromancy is lumped in with summoning Daedra or creating ethereal constructs under the "Conjuration" umbrella. A relevant one of them is that they all share a similar "scaffolding" nature, of creating spatially-bound structures that contain and focus magic. Daedra are given constructed magical bodies to inhabit; bound objects like conjured weapons are pure magic given solidity; and raised dead have an animating principle controlling them almost like puppets, almost directly manipulating their limbs.

This is not the full story, of course. Part of what makes necromancy appealing versus just summoning an ethereal construct or whatever is that it's possible to use the sort of muscle memory that gets embedded in the physical bodies of creatures to improve on direct puppeteering. Between a summoned construct and a raised corpse that are (under some given metrics) identically powerful and capable and smart, the raised corpse costs a lot less magicka and is much simpler to set up. The standardised reanimation spells all have simple ways of making use of the instincts of the dead to make them into more useful minions. So the main way in which these spells vary, really, is in how well they do this, how powerful and/or durable the raised creature is, and how long the spell can keep going before it consumes all remaining life energy in the corpses and turns them to dust.

Now, necromancy that affects souls can be much more powerful, because you're no longer constrained to the limited kind of intelligence that emerges from the combination of whatever is intrinsically programmed into the spell you cast and instincts and muscle memory. Reanimated ensouled dead are just people, kind of. People who are bound to the ones who raised them, and who live inside a body that no longer functions biologically and which is decaying more and more over time, but people. And the presence of a soul means they can use magic, which merely reanimated corpses cannot do. But overall it is terribly unethical to use, of course, since you are not only mind controlling someone but also preventing them from moving on to their afterlife.

(Kind of. It's not forbidden to do soul-affecting necromancy to, say, a chicken. Chickens don't go to afterlives. It's just that, since the techniques used to reanimate and reensoul a chicken are not distinct in kind from the techniques one would use for sapients, it is extremely frowned upon.)

Anyway all of this is to say that the very basic Raise Zombie spell that everyone learns almost as an afterthought at the College creates very weak, very stupid zombies. But the upside is that it is much faster to cast than the kinds of complicated reanimation spells these necromancers tend to prefer, and whenever one of them is taken down they become a weak zombie that will nevertheless be harassing their erstwhile friends from their own side.

It's horrible, actually, from a psychological and emotional perspective. Ruby frankly thinks he'd be traumatised if, say, Onmund died and immediately got raised as a mindless zombie to attack him, no matter how weak he was. He's not sure if these necromancers have built some kind of resilience to this but Ruby would not be okay if something like that happened to him.

"I feel dirty," Onmund says once they're done with this room and are moving on to the next, following the scouting Dremora and vampires. "I understand it's not the same as draugar, but..."

"Yeah, I know what you mean."

"We must press on."

Yeah, they do.

There are a couple more necromancers they run into who are easily dispatched, but eventually they reach another room with some more holding cells and one of them is occupied by an Altmer. "Oh, please! Please let me out of here!"

Vampire?

No.

"Wait—are you from the College? Oh, are you here to rescue me?"

"Orthorn, I assume."

"Yes! So you are here to rescue me!"

"We're here to get the books you stole back, actually."

"...the books? Oh."

"Any idea where they'd be?"

"I, I mean, maybe, but..."

"Oh relax, we're rescuing you, too," says Ruby, walking over to the cage to mess with the lock. Or, well, try to, this one is meant to be mage-proof so Ruby will have to be smarter about it.

"Oh! Okay. Yes, thank you. Uh. The books, yes, the Caller has them."

   Vampire #1 spits on the ground, at that. "I will gut that bitch and drink her dry."

Being "mage-proof" only means that Ruby has to create a conjured key, which is not, let's be clear here, an incredibly easy thing to do, for locks that are at all built competently.

This old abandoned keep's lock was not built competently.

It doesn't take him very long.

"'The Caller'?"

"She's the leader of this little group, the 'brains' of the operation, so to speak."

    "She is the one who has been running those experiments on us," hisses #2. "Our sire is dead because of her."

"And you're in this cage because...? Weren't you joining them?"

"Ah, yes, well, er, I... I objected to some of their practices and, er..."

"I see. Well, consider yourself conscripted, you're helping us kill your friends."

"...they're not my friends."

"Even better."

Scouring the place for loot, as tempting as it is, would be unwise, give these necromancers more time to prepare. So they press on, and move upstairs to the ground floor.

It's been transformed into a makeshift laboratory of sorts, it seems. One large room has several objects one could call operation tables if one were feeling particularly charitable. More like dissection tables, really, or perhaps vivisection, if the minor twitching of one khajiit with their chest cut open to reveal their faintly beating heart is anything to go by.

"...this is monstrous."

"Can you heal her?"