Negative energy damage

From NWNWiki

Negative energy damage results from the negation of life force (positive energy) via its antithesis. As this form of energy directly extinguishes life force, it is typically only effective versus living targets, thus undead and constructs aren't usually affected by its common peripheral properties, such as level drain and ability drain.

Note that while undead are thought to be immune to negative energy since it is this energy that maintains their unnatural existence, exposure to this damage category will still inflict damage upon them. Negative energy spells will only heal the undead target (or any target for that matter) if they are scripted to do so. In particular, prior to version 1.69, negative traps injured undead with negative energy.

Items with negative energy resistance or immunity will reduce or remove negative energy damage altogether, although the latter category of items is more common in the official campaigns.