Downward directed energy deposition into sunspot umbrae may be an important contributor to the energetics of the umbral transition region (TR) and upper chromosphere. Supersonic downflows, and their association with TR bright dots, are now more routinely studied by NASA’s Interface Region Imaging Spectrograph. Observations that trace the impact of these flows into the lower chromosphere and perhaps lower are less frequent. Here we report unique spectropolarimetric observations of a supersonic downflow using the He I 1083 nm triplet observed using the Facility Infrared Spectropolarimeter at the Dunn Solar Telescope. We observe downflows reaching speeds near 200 km/s into a sunspot umbra that present both high-speed absorption and emission profiles. At the downflow terminus, we also observe a sharp transition from supersonic speeds to near-rest speeds, which are connected by extended redshifted line wings of the near-stationary component. Careful analysis, correcting for straylight, and using spectropolarimetric inversions, provide a detailed picture of what we interpret as a strong radiative shock in the sunspot umbra. In particular, we find evidence of very strong magnetic field strengths in the post-shocked flow that suggests the flow penetrates into the deeper umbral atmosphere.