Eternal Chilling Darkness does not trigger off of damage from Death Aura or Negative Energy Burst. This includes damage to surrounding enemies, not just the self-healing effect on a PM in undead form. Also, it doesn't trigger off of these spells even if I drop undead form and I'm not receiving any healing from it.
It does, however, proc off of wail of the banshee, ennervation, and energy drain, assuming they pass spell resistance checks.
At this point, I'm mostly confused. It's working on the opposite kinds of spells I thought it would.