Karma is the force of fate that occultists use to alter reality, learned from studying objects that have been a unique focal point in events throughout history. By studying these objects of power, the occultist gains a fundamental understanding of the myriad possibilities inherent in the oncoming future. (Incidentally, the occultist is also able to judge, by use of the aptly-named "karma" ability, the extent to which he or she has gathered karmic philosophy.) Each time they exercise this knowledge in a display of their occultism, they lose a bit of their foresight and have to study again. The Chaos Lords barter for this knowledge, draining it from the willing in exchange for favours to be had later.
Back before the Burning Times, occultists could only gain karma at an arduous rate, limiting them in power. The precursors for several current occultism abilities were being refined, most notably Shakira's theory that would allow humans to transmogrify into Chaos Lords and Dion de Vermiis' theory of creating a nexus to facilitate and speed up study of karmic items for all occultists. The Shakira Effect sparked the inquisition of the Burning Times, and forced the Ashtan cabal to attempt Dion's Eschaton Experiment.
Under Ezekial's guidance, the Ashtan cabal performed the Eschaton Experiment, tying their karma, and inadvertently, every other occultist's karma, to a single focal point. The experiment failed, and both Ezekial and the created node were lost to a tear in time that the ritual had accidentally created. Only centuries later with Eris and the Occultists Guild was the experiment finished and the Eschaton completed, allowing those with the knowledge of occultism to once again harness karma and practice the lost arts.
In the present age, occultists can also divorce themselves from the group karma the Eschaton embodies, through the intervention of Xylthus. Their study becomes precarious and somewhat unreliable, but they are free to follow their own paths separate from the House of the Occultists.