A Cabalist in action with her mysterious powers.
The Cabalist was the second announced character class, after the Templar. The Cabalist is the game's primary spell caster class, and has somewhat reminded former Diablo II players of its Necromancer class, although this class is more about studying demons than the dead. The class was announced to the fan site community in April 6, 2006, and officially announced in a press release in April 11. 
The Cabal is an organization that studies the beasts of Hell, and their students, the Cabalists, uses this knowledge of their foes' strengths and weaknesses to fight the very subjects to their studies with arcane weaponry and black magic. The Cabalists study their enemies so closely that they constantly balance a fine line between good and evil, which has raised some doubts about their loyalty among other classes fighting on their side.
The Cabal has brought together disciplines of fringe science and ancient, arcane practices to unleash the inner potential of Mankind’s spirit and bring about the next step in their evolution, known as The Awakening. There have always been those who would be prime candidates to become “Awakened”. Sir Isaac Newton was the first modern scientist and the last of the ancient magicians and alchemists. The works of Tesla clearly show that his research went well beyond the common understanding of man. There are some references to Cabalists in turn of the century “majickal” texts by Aleister Crowley and in cult pulp fiction such as Lovecraft’s Necronomicon.
According to ancient Babylonian texts, “The final step in The Awakening is for the Leader, the Great Eye of the Cabal, to pierce the veil and wrestle with the Lord of Hell. If he is victorious, he will gain possession of the Infernal Key of Nine Gates, giving him full command of all the energies from the dark planes.”
Whether or not the Cabalists themselves succeeded in piercing the veil, the forces of Hell have flooded their world. Those that touched the darkness and lived to speak of it were changed. They see the other side of things, often looking at the fate of the world as the metaphor, “The brighter the light, the deeper the shadow.” Although distrusted and misunderstood by most, these seekers of knowledge have shown their final allegiance to mankind on more than one occasion.
The Cabalist was once supposed to have shapeshifting abilities, where one could shapeshift to e.g. a zombie, or various demons. This whole line of skills were however cut, and rumors have also been buzzing about a third class for all factions possibly going to be added post-release as a new subscriber benefit, although this is still speculation.
For skill information, see the Cabalist classes: