  Using the Ernst formalism, a novel solution of vacuum General Relativity was
recently obtained [1], describing a Schwarzschild black hole (BH) immersed in a
non-asymptotically flat rotating background, dubbed swirling universe, with the
peculiar property that north and south hemispheres spin in opposite directions.
We investigate the null geodesic flow and, in particular, the existence of
light rings in this vacuum geometry. By evaluating the total topological charge
$w$, we show that there exists one unstable light ring ($w=-1$) for each
rotation sense of the background. We observe that the swirling background
drives the Schwarzschild BH light rings outside the equatorial plane,
displaying counter-rotating motion with respect to each other, while (both)
co-rotating with respect to the swirling universe. Using backwards ray-tracing,
we obtain the shadow and gravitational lensing effects, revealing a novel
feature for observers on the equatorial plane: the BH shadow displays an odd
$\mathbb{Z}_2$ (north-south) symmetry, inherited from the same type of symmetry
of the spacetime itself: a twisted shadow.
