Baldur's curse is a spell designed by Freya to prevent her son from dying a needless death. Designed to make him invulnerable to any and all injuries no matter how minor or serious, it had the unexpected side effect of removing Baldur's ability to feel completely.
Unable to physically feel anything good or bad, Baldur eventually went insane from sensory deprivation.
Baldur's curse primarily acts as a way to counteract against any wounds and or injuries inflicted during combat. In exchange for the minor ability to feel anything physically, the wielder of this curse becomes an unstoppable force.