Doldrum is a Jupiter Djinni found in Golden Sun: Dark Dawn. Doldrum is found and fought in the dungeon in Craggy Peak. Craggy Peak has 12 hallways leading to 12 puzzle rooms, each themed off one of the twelve in the Zodiac, and Doldrum is in the hallway to the Taurus room.
When Doldrum is unleashed in battle, the user produces an effect that prevents the next one action that the target has not performed yet this turn from executing. If an enemy only has one action per turn, it will essentially be prevented from doing anything this turn, while an enemy that acts twice per turn will only act once. This effect has the "strikes first" property, meaning it will be the first performed action in a given turn regardless of the agility rating of the user and any other combatant in battle.
In Dark Dawn, Doldrum's unleash animation visually resembles the user summoning the 3D model of Doldrum in front of the target. As Doldrum floats around, a glowing purple sphere of electricity forms at the enemy's position, and several transparent "rings" gather into it. The effect disappears as the enemy is overtaken by a puff of smoke and stops its idle animation until the part in the turn when the game reports that it is unable to move.
Golden Sun: Dark Dawn: Doldrum is found and fought in the dungeon in Craggy Peak. Craggy Peak has 12 hallways leading to 12 puzzle rooms, each themed off one of the twelve in the Zodiac, and Doldrum is in the hallway to the Taurus room. Enter this hallway, hop up once to the right, and cast Grip on the white spire to rotate a stairwell counterclockwise so that the stairwell connects to an elevated platform to the left. Hop right and up, and use Grip on the other stairwell to rotate it counterclockwise and make the stairwell connect north. You can now hop to the stairwell, climb it, and hop left to the elevated door to the Taurus room, but you will see Doldrum in the jaws of a lion-head statue to the left. Cast Fireball on it to make it drop to the platform below. Retrace your steps back to the hall's entrance, and go up to the elevated platform to the left and follow its path north to where Doldrum now resides. Fight it to earn it, then continue to the Taurus room to progress with the dungeon. As Craggy Peak becomes permanently inaccessible once Belinsk Ruins is completed much later in the game, Doldrum can be permanently missed by that point.
General: Doldrum is one of several Djinn that can either nullify an enemy's action entirely or prevent one of a boss' multiple actions in a given turn. This can be potentially very useful, and specific applications are possible when this Djinni is combined with the other Djinn that has the same effect (so that even a boss' multiple actions can be negated in a turn). GBA equivalents to this Djinni are the Venus Djinn Ground and Petra.
When Doldrum and another Djinni with an identical effect are in your party's possession, the two together can completely prevent any enemy that only acts once per turn from ever attacking. While Adept One uses one of the two Djinn to freeze the enemy, Adept Two re-Sets the other of the two Djinn that was used to freeze the enemy in the previous turn, and the next turn Adept Two does the freezing while Adept One resets his Djinni. Adepts Three and Four, in the meantime, are the fighters who safely deal damage. This alternating strategy can be infinitely performed to ensure complete safety for the Adepts throughout the battle (though of course the battle would be a lot longer and more tedious this way). This is not as insanely helpful against any boss that can act more than once in a turn, though, since its second attack will not be prevented.
Golden Sun: Dark Dawn: For most of the game, Doldrum is the only effect that prevents an enemy's action entirely, and does so as the first action of a given turn when it is selected for use. This is different from the Stun status condition in several important ways: It always works, and it has the top priority of any move in a given turn so it will always hit first, but it will only negate one of an enemy's actions instead of preventing it from acting at all for potentially several entire turns. Typically, though, random enemy fights should be easy enough that preventing them from acting with Doldrum won't be as useful as simply attacking, and this goes for several boss fights as well. This makes Doldrum not so useful despite having a technically powerful effect.
Near the end of the game, the party can get Doldrum's identical counterpart, the Venus Djinni Ivy, and the end of the game is when these Djinn can become useful. Endgame boss fights such as the final boss and the optional bosses can be made significantly more manageable by having one of their multiple actions in a given turn negated. In these fights, enemies can spend their turns performing dangerous and strong effects, so preventing one of them is a good way to use up the turn of one of your four battling Adepts. Another especially under-appreciated use of this effect manifests in the start of the Star Magician battle; Doldrum and Ivy share the same priority with the Guard Aura monster skill the Guardian Ball uses to make the Star Magician invulnerable with, so if the user has a higher Agility than the Guardian Ball (if its agility is too high, this problem can be solved with agility-doubling Djinn like Fleet or enemy-agility-halving Djinn like Vine), the user can outspeed the Guardian Ball and make it unable to use Guard Aura with Doldrum/Ivy, thus ensuring that the other three Adepts this turn can fully and safely make use of the strongest summons all together. This Djinni can singlehandedly make a tricky optional boss battle much more simple than otherwise possible, in other words.
Doldrums are regions of light baffling and calm winds in the convergence zone, which spread along the northern side of the Equator.