  We propose a simplified model to study the possible new heavy diphoton
resonance from cascade decay of a heavier particle at colliders, which may be
related to the dark matter or other new physics beyond the standard model.
Model-independent constraints and predictions on the allowed couplings for
generating the observed diphoton data are studied in detail. We demonstrate
that this scenario can be tested by the possible four-photon signal or the
$WW/ZZ/Z\gamma$ resonances. Meanwhile, this cascade decay scenario also
provides us with the dark matter candidates, which is consistent with the
observed dark matter relic density.
