  Causal concept for the general black hole shadow is investigated, instead of
the photon sphere. We define several `wandering null geodesics' as complete
null geodesics accompanied by repetitive conjugate points, which would
correspond to null geodesics on the photon sphere in Schwarzschild spacetime.
We also define a `wandering set', that is, a set of totally wandering null
geodesics as a counterpart of the photon sphere, and moreover, a truncated
wandering null geodesic to symbolically discuss its formation. Then we examine
the existence of a wandering null geodesic in general black hole spacetimes
mainly in terms of Weyl focusing. We will see the essence of the black hole
shadow is not the stationary cycling of the photon orbits which is the concept
only available in a stationary spacetime, but their accumulation. A wandering
null geodesic implies that this accumulation will be occur somewhere in an
asymptotically flat spacetime.
