The Harbinger of Nightfall does have natural boss health regeneration, but as its HP are prevented from changing in any way unless it is hexed with Vial of Purified Water, this (as well as healing from its Margonite allies) will only make a difference while the holy water affects it.
Jennur's Horde mission
In this mission, the Harbinger of Nightfall can only be damaged by the Lights of Seborhin.
Unlike the normal Harbingers, this one only loses 1/3 of its health to a single Light of Seborhin, requiring 3 Lights to kill it completely.
The Harbinger of Nightfall has no inherent health regeneration, and any healing skills used on him will be nullified.