Dark Magic or otherwise known as Black Magic is basically the dark shadowy version of magic itself. It is mostly the practice of bending the laws of physics through sheer will,specific eldritch words and spells,and strange inter-dimensional energies that coincidentally are in the normal universe.
Dark Magic is usually associated with things like death and necromancy and is usually described to have a purple eerie glow in its purest form. The most common trait in dark magic is that it usually backfires spectacularly for those not skilled enough to use it, or if the user just has terrible luck.
Depending on the strength of the magic and the skills of its user, dark magic can be a force to be reckoned with.
In its solid form Dark Magic is an eerie purple volatile substance that can generate "dark energies". These energies can cause nightmarish mutations in any unprotected lifeforms unlucky to be nearby. These mutations usually cause loss is sanity a and possible biological immortality. That or a person can get "infected" if they stay near it too long even while protected.
Once a being is infected a dark shadowy version of the being will begin to form in their body. This will cause the infected person to consume as much energy and nutrients as possible to feed the energy being now inside them. Once the being is developed enough it will take control of the infected persons body. Soon the copy will eventually separate from the infected and become a physical entity with natural black magic abilities.
At this stage the infected person will most likely vomit uncontrollably and have a weakened immune system for a few weeks.