Nightwing

[Doppel]

Does it boost damage from cold damage charms or items like duress etc,...?

[NASE]

The mod on nightwing is a mastery-mod. This means that all skill damage is increased.

Elemental damage from charms and items - and duress shouldn't be different - is only increased when applied mele. So if you plan on using this helm with a frostmaiden, the elemental damage from charms isn't increased.