When fighting this creature you will have to hex it with Vial of Purified Water, which is a projectile hex, in order to do any damage.
Also, it is advisable to bring at least two human players as the vial only lasts for 10 seconds but has a recharge of 15 seconds.
You can also use Serpent's Quickness to recharge the vial faster, thus making him permanently vulnerable.
While not hexed with Vial of Purified Water, this creature's HP is prevented from changing in any way. It does have the usual +3 pips of health regeneration that all bosses have, but it will have no effect except during the time the Harbinger is affected by the holy water. This also prevents the Harbinger from recovering his HP through natural health regeneration if disengaged.
Although they are not bosses, killing a Harbinger of Twilight will provide a Morale Boost.
Skills such as Nightmare Weapon will still steal health, even if they are not under the effects of a vial, although they will heal the damage moments later.