Curse is a Venus-based support Psynergy that has a chance to inflict the target with a Death Curse, giving the affected person/monster seven in-battle actions before being automatically downed. Death Curse's counter goes down by one every time the afflicted character performs an action or an enemy successfully uses Curse on them. Curse is always healed when a battle ends, or when a character is downed. Curse costs 6 PP to cast and is available at level 24 to the Dark Mage and Brute class series and at level 20 to the Medium and Apprentice class series, and is thus available to all Adepts.
Considering the quick nature of most battles, the target will likely be downed before the instant kill takes effect, making its usefulness limited in random battles. Any boss fight in which the boss has a Luck rating 40 or above is also out of the question because the boss is immune to the stronger status conditions at this point. However, in the first Golden Sun, most of the bosses don't have luck ratings anywhere near as high as 40, meaning that Curse can be put to great use against the Kraken, Toadonpa, Storm Lizard, and Tempest Lizard to make those battles end in no more than 3 turns.
Aside from those impressive niche roles Curse can fill, its low hit rate and lack of other applicable targets lowers its usefulness otherwise, in which case it might be considered more useful when used by enemies. However, this is again unimpressive, for unless you're using the Jupiter Djinni Kite on an Adept that is afflicted with Death Curse, the Adept would have to take seven turns to act seven times before the instant death effect triggers. Random battles in Golden Sun games go by very quick because enemies such as the Doomsayer monster line (one example of monsters that have Curse) are easy to kill quickly in just a few turns, and the Death Curse effect always disappears from an Adept once a battle ends, so rarely will it prove to be not a wasted move by the monster involved. The only possible way Curse can become a concern when used against you is when a boss uses it, since boss battles take a lot more turns to beat. However, as is the case with the optional super-boss Dullahan, sometimes a boss is much better at downing your cursed Adepts quickly with their other attacks before their Curse timers are up.