Light-powered engines

The contraption looked like a weird mix between a gramophone and a living thing. At its top, there was a fleshy tube that got narrower towards the rest of the device. It was connected with some pulsing cables to the metal box.
"So this is the engine?"
"Yup, standard model. One level of light per hour of it working. You turn it on here," the owner of the shop flipped the switch. The machine started moving a little and the lights in the shop started to flicker and became a little dimmer. At the same time, the box on the bottom started buzzing with electricity.
"Okay, I'll buy three."

In the Dark Halls, the light is the most important resource. It protects from the hunger of the darkness that can take away everything, both material and immaterial. And to use it as a power source could be either risky or insane, depending on one's location. And yet, someone built devices that do exactly that.

Fully organic machine

Like a lot of other inventions inside the Dark Halls, all of the parts used in the creation of the light-powered engines are organic, coming either from the creatures living here or products made out of what darkness stole from other people.
The engines require only three components to work, but if someone wants to change the machine's efficiency, the amount of energy created, or the size of the area from which the engine siphons the light, they have to add more parts that will influence those parameters. Those are the three required ones:

1. Membrane of a Light-Thief

As their name suggests, Light-Thieves are creatures capable of consuming and extinguishing the light. Researchers found out that if the right electrical charge is sent through their bodies, then even after their death, their mouth membranes still capture the light.

2. Nerves

It doesn't matter if they are extracted from one of the creatures or from one of the Halls that steal human nerves, as long as they still function. Nerves serve as an activator for the membrane, and a transmitter and converter of light into a more universal power source (the light engines work with electric devices and ones that require combustion of fuel).

3. Energy box

Energy boxes are made by extracting electric glands of sparks and encasing them in faraday cages. This prevents the glands from spilling the power outside and instead makes them store it. Thanks to the connection with the nerves, a small charge can be sent toward the membrane to activate the engine.

How does it work

Light levels:
Blinding -> Aureate -> Incandescent -> Bright -> Warm -> Glowing -> Dim -> Fading -> Penumbral -> Dark -> None

The light-powered engines consume one light level every hour, making the area darker and darker. Once they are turned off, the light sources need the same amount of hours as the number of light levels they lost to regain their full power. So a light source that can only produce Dim light will at most need 4 hours of regeneration, even if the engine will work for longer thanks to the other, stronger light sources in the area.
The light consumed by the light-thief membrane is turned into energy by it. The nerves connected to the membrane collect said energy and turn it into electric pulses. The pulses are both stored inside the electric gland and stimulate the production of even more electricity by the said gland.
Each energy box can only store a certain, usually small amount of energy. That's why the engine always should be connected either to the devices it has to power or some kind of battery to prevent the waste of power.

Social Impact

Light-powered engines brought electricity to everyone in the Dark Halls. Before, only those lucky enough to acquire a generator from one of the First Halls or those who knew how to produce some power out of the things found in this dimension had the access to it. But now, as long as the engines are kept in a safe space, people can gather or craft and use many devices from our world. Lights that require less or almost no fuel, vehicles, and automatic defenses are only a few things that made parts of Dark Halls either safer or more accessible.
The discovery of this technology pushed many inventors to experiment with the parts of the creatures found in the shadows and with the things stolen and processed by the darkness. Machines, vehicles, tools, and weapons, that could be impossible to create in our world become a reality here. And maybe, at some point, one of them will let people return to their homes.

Complexity

As long as one has the three main components and a general idea of how to connect them together can create one of the engines. It's when one wants to add more specific components to improve the engine in some way that the task gets way harder.

Discovery

No one remembers anymore who was the first inventor of the light-powered engine, they were probably consumed by the darkness, whether while still alive or after they have passed away and were forgotten. What is known is that the first-ever models started showing up in the 20s. From that time, their blueprints spread, and now almost anyone who can read them and has some common sense to not experiment with them too much can create their own engine.

Spread and utility

Since both the gathering of the components and the construction are pretty easy, the engines spread all around the Dark Halls. And while they can't be used in many places due to safety reasons, they still find their use mostly on caravans, where lights can be quickly repaired and travelers can rest while their light sources regenerate, or in areas that are inaccessible by common people.

Caravan was racing down, chased by a giant centipede-like creature.
"Change the bulb! It'll catch up to us if the light level drops more!"
"Sir, but that means being exposed to the darkness!"
"You prefer losing a finger or dying here?"
"Sir, I..."
Their captain didn't listen to them, anymore. Instead, he grabbed the lightbulb and entered the cart that was almost completely dark. Not a minute later, the caravan sped up and the room was well-lit again. The captain returned, two fingers missing from his left hand.
"This should buy us four hours of driving."

Risks and drawbacks

The most obvious risk of using light-powered engines is their consumption of light levels. This process has to be monitored, otherwise one could quickly end up with no light source strong enough to stave off the hunger of the darkness inside the Hall. Once in a while, either an empty caravan or one full of bodies can be found in various parts of Dark Halls, just because its members forgot to pay attention to the lights around them.
If not maintained properly or subjected to strong shaking or physical damage, the energy box can explode, releasing all the power inside of it in the form of sparks, lightning, and even an electromagnetic wave shocking everything in the area. This can, in best cases cause fires and stun people for some time, and at worst, kill them. That's why the bottom parts of the engine are often reinforced and surrounded by materials that could possibly absorb the released electricity.