Curse

From NWNWiki

A curse is a negative effect that reduces ability scores, and that is notoriously hard to be rid of. Most curses have no time limit, and they are not removed by resting. The only spells that have an effect on curses are greater restoration and remove curse. In the absence of these spells, the only way to remove a curse is through a module-specific feature. (Many modules offer "temple healing" that will remove a curse.)

The most notably source of a curse is the spell bestow curse.