  The magnetic excitation spectrum of the quantum magnet YbCl$_3$ is studied
with inelastic neutron scattering. The spectrum exhibits an unusually sharp
feature within a broad continuum, as well as conventional spin waves. By
including both transverse and longitudinal channels of the neutron response,
linear spin wave theory with a single Heisenberg interaction on the honeycomb
lattice reproduces all of the key features in the spectrum. In particular, the
broad continuum corresponds to a two-magnon contribution from the longitudinal
channel, while the sharp feature within this continuum is identified as a Van
Hove singularity in the joint density of states, which indicates the
two-dimensional nature of the two-magnon continuum. We term these singularities
magneto-caustic features in analogy with caustic features in ray optics where
focused envelopes of light are generated when light passes through or reflects
from curved or distorted surfaces. The experimental demonstration of a sharp
Van Hove singularity in a two-magnon continuum is important because analogous
features in potential two-spinon continua could distinguish quantum spin
liquids from merely disordered systems. These results establish YbCl$_3$ as a
nearly ideal two-dimensional honeycomb lattice material hosting strong quantum
effects in the unfrustrated limit.
