Based on the comments to the question, it seems like the question is really about:
How could an entity gain some kind of energy in return for causing people to see the world as dark in some sense?
I see multiple ways that this could be approached:
- There is some kind of bounty available for spreading visions of darkness.
- Visions of darkness erode the barriers between realities, making it easier for the villain to access reservoirs of their chosen energy source.
- A suitably trained person could choose to perform seemingly supernatural feats whenever they like, with the understanding that this induces morbid visions in people who are close to them (either spatially or socially).
Option 1: It is a bounty!
The villain has an arrangement with some other power/market/mentor who can grant energy. The more people who the villain can corrupt with visions of death and darkness, the more energy is up for grabs, according to the market agreed price.
In a narrative, this could manifest in a lot of different ways. It might be well suited to narratives intended for exploring themes of greed and corruption.
Option 2: The visions are real!
There really is some kind of "bad" universe out there full of cool dark energy ready to be used by the enterprising sorceror. Think of the "upside-down" in Stranger Things, or the Negative Energy Plane in Dungeons & Dragons, or the Dark Dimension in Marvel.
The villain is at home in
<insert bad place name here>. That is where they get their energy. When they are at home, no problems. But now they are visiting planet Earth, how can they get their sweet nectar? Easy: erode the barriers between dimensions.
Through whatever means, the villain is able to induce morbid ideation in regular humans which actually reveals visions of the villain's homeland. Seemingly harmless (albeit unsettling) these visions are actually true and show how things really are in that bad place. Recipients of these visions could go insane, but they also gain insight into the true nature of reality; but the villain doesn't care about all that, all that matters is that these visions make it easier to "phone home" and grab some sweet energy drinks from where they are kept cold.
This option would be good if you're going for a kind of creepy Stephen King/Stranger Things atmosphere.
Option 3: I ain't the bad guy here!
The (so-called) villain discovered that they have some special power which they can freely use whenever they want, but it has an unfortunate side-effect: it induces morbid visions and feelings of darkness in the people around them. When should they use their power?
If they save someone, does it matter if that person is irrevocably changed, so they can no longer be friends with the person who saved them? Or, a person is saved, but maybe the effect of the darkness induces them to do something drastic, such as committing suicide, or becoming a dangerous criminal?
This option would be suitable for a noirish or angstish storyline.
Let your narrative direction drive the pros and cons of any powers used, and any drawbacks imposed for those powers. If you choose the power and drawbacks in advance, that will prove to be neither sufficient nor necessary for exposing the theme and plot that you're actually interested in.