The first sign of life that they find is a room with four cages in it, three of which are occupied. "Oh, are you the reason for the commotion?" one of the prisoners asks.

"...probably. What was the commotion?"

"No idea. We just heard noises and then someone came here and closed the door." She gestures at the door they didn't come through.

"Were they planning to experiment on you?"

"Naturally."

"Careful. These three are vampires."

...ah. Yeah, that would make sense, wouldn't it. "I assume the cages are magical, then?"

    "Yes," says a second vampire. "Sun-powered."

"Here's a deal," says the first. "Free us and we'll help you slaughter those insolent mages."

"Do not do that," says Erandur. "Even if they hold their end of the bargain and don't immediately turn on us they are still vampires and can cause untold suffering and death if let loose."

"They would be great help against the necromancers, though."

"They've already lost once to the necromancers, they cannot be very powerful."

"They caught us unawares," says vampire #1, "and ambushed us while we were separated. If we are together they will not stand a chance."

        "It would be stupid to renege on the deal," says vampire #3. "If we act dishonourably then we will not be able to be trusted in future deals."

    "We weren't even bothering anyone, we were just hanging out," whines vampire #2. "We're not a big powerful coven that keeps trying to attract attention and get into fights."

"Promise you won't kill any innocents."

"...what, today or..."

"Ever again."

    "Come on!" says #2.

        "Acceptable," says #3.

"Is it?" asks #1.

        "The alternative is death." He looks at Ruby. "Is it not?"

"Oh, yeah. I'm not leaving you guys here. Either you make the promise and I let you out or you don't and I kill you. You have five seconds to decide."

"...fine."

    "Ugh fiiiine."

"You would trust their word?"

Ruby shrugs. "I will trust my impulse to mercy. And if I find they betrayed their word, ever, I will burn them. I'm also planning to be immortal and extremely powerful so I'm sure I'll be better at tracking them in the future than they'll be to flee."

As he says this, he walks over to the cages to analyse the magic, determines that there is nothing preventing a brute-force approach, and opens them one by one.

    "Now die," drawls vampire #2.

        "Arabella this is not the time for your jokes," sighs #3.

"I thought it was funny," says #1. "Anyway, thanks for that, presumably you don't think the necromancers are innocent?"

"If any of them surrender or don't want to fight don't kill them but otherwise good luck have fun."

"Excellent." She leads the way and kicks the door open. "Come at me, assholes," she calls into the room on the other side, and... they oblige, actually.

Good thing vampires have good magic resistance.

...alright. Ruby has the Dremora walk ahead into the other room for some support and then onwards they go.

There are six mages in the other room, plus four skeletons, one zombie, and three conjured Atronachs.

This is, actually, not at all enough to deal with three vampires, Ruby, the Dremora, Onmund, Onmund's Potent Frost Atronach, and Erandur, who won't attack the mages themselves but has no issues attacking their constructs and raised dead.