Edge-on impact experiments on AlON, a transparent ceramic with superior ballistic performance, show combined intergranular and transgranular fracture. Following the impact event, a fast-propagating damage front advances through the sample and eventually transitions into a slower-propagating localized cracks. We present a peridynamic model for polycrystalline AlON to investigate the failure evolution observed in experiments and understand why and how the damage front moves at supershear speeds. We use a computational polycrystalline structure with the same average grain size as the samples used in experiments. The peridynamic model helps explain the reasons behind the observed failure front supershear propagation speed (higher than root 2 times the shear wave speed), and the subsequent transition to sub-Rayleigh propagating localized cracks. The computed propagation speeds for the damage front, and the cracks that emanate from it, match very well those measured in experiments. Elastic anisotropy, material microstructure, and brittle failure are the only ingredients used here to determine damage evolution in edge-on impact on brittle polycrystalline materials, under no confinement. Other possible dissipation mechanisms, like twinning, plasticity, friction, are not included in the model. These are likely second-order effects in the evolution of damage and failure for this material and at these impact speeds.