Neutrino lines and photon continua from cascade dark matter decay

We investigate the two-body decay of fermionic dark matter, $\chi(\bar{\chi})\to X+\nu(\bar{\nu})$, where the light mediator $X$ subsequently decays into photons. We consider two benchmark models: an axion-like particle with $a\to\gamma\gamma$, and a kinetically mixed dark vector with $A'\to3\gamma$. This decay topology produces a monochromatic neutrino line from the primary decay together with a broad secondary photon continuum. A key feature of the scenario is that the photon signal depends on the mediator decay length, whereas the neutrino line is produced promptly and is insensitive to the subsequent propagation of $X$. We derive dark matter lifetime limits from current and projected MeV gamma-ray and neutrino searches, including both Galactic and delayed extragalactic photon contributions. We find that photon constraints generally dominate for short-lived mediators, while neutrino-line searches can become competitive or provide the leading sensitivity in regions where mediator propagation substantially suppresses the photon signal. This conclusion remains stable under conservative extragalactic-only limits and a simplified treatment of Galactic spatial smearing.