I was reading earlier today.
Seems that the dmg proc and the slow proc might be separate and might have separate rules regarding the cooldown for melee vs spells.
For melee there appears to be no cooldown and the proc is based off weapon speed, however for casters the proc chance is ~50% and the cooldown is estimated in the 20s range.
Does not proc off consecrate or any other dots/channeled effects.
It does not stack with TC or CoT and I assume Slow, however it may be possible to apply the debuff to certain mobs that are immune to magic such as the council members
It's the thought that counts, not how many people die because of it.