Dark magic is inherently Chaotic and its purest form is reserved for the followers of three of the four Powers of Chaos: Tzeentch, Slaanesh, and Nurgle. Not all users of Chaos magic are wizards who have been turned by their desire for power; some are cultists who have been granted spells by their Chaos God or schooled in its use by Daemons or by other cult members. The spells of the three gods are very different and there is intense rivalry between them. Moreover, because there is so little communication between cults of the same god, each one may have developed particular spells and magical techniques unique to them.
Chaos magicians are, like all users of Dark magic, intensely secretive about their art, for to be found out is certain death. Some have discovered that the best disguise is in a reputable member of one of the other schools of magic: that way they can even acquire a licence. All the colleges are wary of this potential deception, but some are better at detecting dubious members than others.