  Cuspy shadow was first reported for hairy rotating black holes, whose metrics
deviate significantly from the Kerr one. The non-smooth edge of the shadow is
attributed to a transition between different branches of unstable but bounded
orbits, known as the fundamental photon orbits, which end up at the light
rings. In searching for a minimal theoretical setup to reproduce such a salient
feature, in this work, we devise a toy model with axisymmetry, a slowly
rotating Kerr black hole enveloped by a thin slowly rotating dark matter shell.
Despite its simplicity, we show rich structures regarding fundamental photon
orbits explicitly in such a system. We observe two disconnected branches of
unstable spherical photon orbits, and the jump between them gives rise to a
pair of cusps in the resultant black hole shadow. Besides the cuspy shadow, we
explore other intriguing phenomena when the Maxwell construction cannot be
established. We find that it is possible to have an incomplete arc of Einstein
rings and a "fractured" shadow. The potential astrophysical significance of the
corresponding findings is addressed.
