Unlike the Light, black magic, also called dark magic, is not a singular cohesive energy; rather, it is a collective of magical styles and types that center around using pain, fear, shadows, chaos, and demons. Black magic has no intelligence or directives; despite popular propaganda, black magic does not inherently seek to destroy. Black magic is simply a tool, not a motivation, and can be used neutrally. However, the stigma against black magic and cults/cultists is immense and pervasive in nearly every culture on every world, not just among Lightworkers, and as a result tends to draw the most unstable and dangerous individuals to the craft. It is assumed that anyone who uses black magic is an active part of some cult, even if the individual is alone and unconnected.
Black magic is often symbolized by black fire, blood, or corpses. Black magic uses induced pain and terror against targets, often able to drive a person to the brink of insanity. Shadows are usually used for cloaking and stealth, as they have little practical use in direct combat. Chaos is a rarely-harnessed ability that is not actually a type of magic, but a force that unravels and scatters magical efforts in its vicinity; it's most useful against other magic users, but it has no effect on the Light. Chaos is usually used in small pieces to weaken an existing spell, often a protection or warding spell; too much can disrupt a dark magic ritual and ruin the effort. Black magic also can be used to imbue a target with a physiomagical disease or plague that eats at the mind and spirit as much as the body.
Black magic encompasses but is not limited to demonology. In essence, all demonologists use black magic, but not all who use black magic are demonologists. Even warlocks, the most 'pure' black magic caste, can choose not to study demonology.
Demons are semi-corporeal creatures of variable intelligence and unlimited shape that reside in the void outside the suns; their hells are onion-like layers of space surrounding the layer of worlds in the center of the universe. The closest hell is the first hell, and the farthest is the 99th or 100th, depending on which source you use. The middle 50th hell contains the most powerful and ancient demons, while the demons in the first and 99th/100th hells are the smallest and weakest. Demons survive by consuming magic, which they can get from other demons as well as physical bodies; while they do not need to kill a person before devouring its magical essence, they're usually trained by their summoners to do so. The only way to kill a demon is to consume or completely destroy its magical energy; destroying its semi-physical body only unsummons it. (You can read more about the technicalities of demons and the void on the universe mechanics page.)
Would-be demonologists learn perfect self-control in the face of all emotions and physical stimuli before being taught how to summon and bind demons to their service; demons will attack upon first hint of pain or fear. At least half of all novice demonologists die in their first summoning attempt. Only warlocks can call demons from the twentieth hell; for all others, the limit is the tenth hell. (Note: While the first and 99th/100th hells are technically equivalent in power, the 99th/100th hell is too far to reach via summoning.) Some demonologists treat their demons as partners, a win-win situation where a demon can eat and grow stronger while their summoner kills its targets, while other demons are treated as defiant slaves or pets. The more magic a demon eats, the more powerful it is for a time, so frequently-summoned and -fed demons are quite hefty; most demonologists form a relationship with a singular demon and work with it to strengthen it and increase its usefulness.
With very few exceptions, every person who uses dark magic is a member of a cult or was trained in one, so 'cultist' is used as the term for a practitioner, instead of 'black mage', which is a particular type of cultist. (See Classes below.) Cults are groups of black magic practitioners, independent of any particular world or culture, almost always multispecies. They are driven by kill-or-be-killed attitudes, led by the strongest and deadliest, and secretive even within their own ranks. Most cults also serve as academies and training grounds for new recruits and novices, using a sink-or-swim approach in most things. Many beginners are killed in progressing through their courses and ranks. Cults vary widely in structure, size, and purpose; some are anarchist collectives of individuals who have no unifying goal, while others are well-oiled machines of destruction. Most cults cannot be destroyed or scattered by killing their leaders, but since cultists tend towards privacy - masters take apprentices, not hold classes - a cult's collective knowledge and competency can be greatly decreased by offing the best of its members. While a cultist can leave one cult and join another, cultists are usually quite clannish and will often make efforts to kill those who leave.
The following descriptions were taken from the human magic classes page. They are reproduced here for convenience.
Warlock (pure) - demonology, disease/plague, curse/hex, debuff, black fire, shadow, pain, fear
Witch Doctor (warlock + druid) - disease/plague (via concoctions), curse/hex, debuff, animal aspects, communion with nature & spirits, elemental magic
Black Mage (warlock + mage) - demonology, curse/hex, black fire, shadow, spells, rituals, psyche, power storage & management
Necromancer (warlock + priest) - curse/hex, debuff, shadow, pain, fear, control and resurrection of the dead, communion with ancestors & gods, life transfers (anti-healing)
Banshee (warlock + warrior) - disease/plague, curse/hex, black/red fire, fear, pain, rage, physical prowess, qki
Shaman (warlock + druid + warrior) - curse/hex, elemental magic, debuff, limited communion with nature & spirits, physical prowess, protection/guardian, rage, qki
Assassin (warlock + mage + warrior) - black fire, shadow, spells, psyche, physical prowess, weapons expertise, qki
Reaper (warlock + priest + warrior) - curse/hex, pain, fear, control of dead, physical prowess, weapons expertise, rage, qki
The following lists are rough estimates only. From top to bottom are the most to least populous species found in varying cults; each species is then followed by its typical class, from most to least common.
human - all cultist classes
daereon - warlock, black mage, necromancer
b'kanna - witch doctor, shaman
anean - all cultist classes
xaol - black mage, assassin
sirian - black mage, assassin, necromancer, reaper
vur - warlock, banshee, necromancer, reaper
k'anta - black mage, assassin, warlock
ret'chek - witch doctor, shaman
rarra - black mage, assassin
caline - witch doctor, shaman
gracken - black mage, witch doctor, assassin
ku Lo'hês - witch doctor, shaman