Qlippothic Sphere is a chaotic counterpart of one of the usual nine Spheres of Creation. Each Sphere has a Qlippothic opposite swirling with dark resonance. They are usually employed by the Nephandi, but there are other mages who have been drawn to their power.
While normal Spheres operate by creating, changing and rearranging elements of reality, Qlippothic Spheres operate by reality's destruction. A normal portal might temporarily "sew" together two places in the Tapestry, but a portal created through Qlippothic magic would literally tear a hole in space, negating the space between two locations. Each Sphere has its own associate Qlippoth.
All mages try to overcome themselves and the shackles of reality in one way or another. The Nephandi - however - take this idea to the extreme. To them, the ultimate obstacles to overcome are the taboos and morals of the human condition, adherence to sanity, and the very existence of reality itself. They do not intend to achieve a higher form of being i.e. Ascension, but strive for utter destruction and oblivion - Descension, as it were.
As a result, even their view on reality is so twisted and warped that they cannot use the usual nine Spheres, but have to adhere to a understanding of all things that are, informed by the way that they fall apart rather than by the way they are put together. This is called by the tradition mages the Qlippoth of a sphere, a twisted, eroded picture of what things should be. Therefore, Nephandi magic differs from normal Sphere magic in ways more profound than paradigm alone: they use the mirrored opposite of the powers of the Spheres to work their arts.
They also have a different relation to Paradox than conventional mages. Because their magic aims to disrupt, rather than simply change reality, Paradox tends to strike them harder than conventional mages and they can therefore achieve only so much in terms of unraveling reality. As a result, Nephandi have become very skillful of keeping their workings hidden and subtle.