Let me share with you some lore that I created for my Villain. In reality I wanted it to be an anti-hero, but due to it's grotesque design and the source of its superpower/ability it leans more towards Villain.
First of all, Oskuro is an ethereal creature that in its purest form looks like a ghost/specter/Dementor (Harry Potter reference). The main goal of an Oskuro is to being able to survive on its own, but when they are on their weakest form they need to live in a contained environment. In a weak state they don't have reflections, but this changes when they get stronger. They used to take over human/animal hosts but sharing a body with another entity made it difficult to being always on control of the body. This is when Oskuros decided to give up on organic life and moved on to controlling synthetic objects.
Once an Oskuro takes over a synthetic host, it wanders around dark alleys, streets, and places with minimum lighting hunting for prey. Oskuros tap into its prey's soul, mimic their inner fears and/or phobias, understand them, and ultimately shape-shift into them. They use fear to increase in power and vitality. Oskuros can even shape-shift into abstract fears which make them very problematic. In order for Oskuros to keep their fear form, and don't loose the power that comes with it, they eat/absorb their victims which makes them dangerous to fight against.
Oskuros might look very dangerous, and they are, but they have three weaknesses. One, they are afraid of fire and their ethereal bodies can't sustain much fire damage before crumbling and disappearing into nothingness. Fire damage works great when Oskuros are on their weakest forms, but when they are stronger they get minimum damage.
Two, if you get rid of their contained environment they loose strength, which ultimately makes them vulnerable to fire, or with time and no other containment, they will die by themselves. The problem comes when they reach their strongest forms and they get rid of their containment.
At this point the only way to defeat an Oskuro is by using a mirror. Previously in their weak form, Oskuros don't have reflections, but when they get stronger they grow a pseudo soul by eating/absorbing their victims. If the hero manages to reflect the whole Oskuro's figure, the Oskuro will go crazy by looking at fear in its purest form, this is the best moment to attack it with fire and kill it.
