[Gem Mountain - North]

The hallway's lights continue to dim until the only lights are the lights the two are using, and the lighting from the rooms behind them.

Rein is almost entirely correct in his assessment of the cause.

Once the lights are out, the charcoal-y breeze that has been coming down the hallway begins pulsating, like it's being pushed by a heartbeat.
Not all of the bodies start to move, only three of them. Although they're not technically zombies, I'm going to call them zombies for the sake of convenience.

The three zombies stand up. It's not in the manner you would typically associate with standing up. Instead, it looks like they were puppets attached to invisible strings, and the puppeteer is pulling the strings to make them stand. They start stumbling towards Rein and Tina, again as if they're puppets walking, without a care for what's the normal way to walk. And then, suddenly, the closest zombie's open mouth begins to glow with light, which seems to have an origin in the back of its throat. Some thick green gas fills its mouth.

Then
Like a flamethrower, a plume of fire blasts out of the first zombie's mouth towards the two adventurers, and the fire stops after three seconds.

And the second zombie's mouth starts to glow in the same manner.