  We present an exact solution of Einstein's equation that describes the
gravitational shockwave of a massless particle on the horizon of a Kerr-Newman
black hole. The backreacted metric is of the generalized Kerr-Schild form and
is Type II in the Petrov classification. We show that if the background frame
is aligned with shear-free null geodesics, and if the background Ricci tensor
satisfies a simple condition, then all nonlinearities in the perturbation will
drop out of the curvature scalars. We make heavy use of the method of spin
coefficients (the Newman-Penrose formalism) in its compacted form (the
Geroch-Held-Penrose formalism).
