Surge is a Mercury Djinni found in Golden Sun: Dark Dawn.
When Set, Surge increases base HP by 11, base Agility by 3, and base Luck by 2.
When Surge is unleashed in battle, all non-downed Adepts in the current battling party are cured by 30% of their maximum HP meters.
In Dark Dawn, Surge's unleash animation visually resembles the user summoning the 3D model of Surge above himself or herself, and it disappears in a blue flash shortly afterward. Then, a collection of light blue energy orbs spin and gather into each receiving Adept in order, similar to the Wish Psynergy series.
Golden Sun: Dark Dawn: Surge is found deep inside the Barai Temple dungeon in Ayuthay. Late in the dungeon, you will pass by it on a small, elevated platform above a pond. You can get it right away by casting the Fireball Psynergy on it so that it jumps down into the pond below. Then, cast the Arid Heat Psynergy on the water-filled bowl to empty the pond and get to Surge, and it must be battled to be obtained. Alternatively, you can skip past this for now and reach the end of the dungeon, where you obtain the Insight Glass in a cutscene and use that to get back. After this sequence, when you re-enter the room an extra cutscene with Amiti will prompt you to use the Insight Psynergy while the Djinni is on the screen, and it will show you how you need to cast Fireball on it. Get Surge at this point.
General: There are very few healing effects in any of the games that restore HP based on a percentage, and such effects that heal the whole group are rarer. What primarily causes Surge to fall into obscurity is the presence of group healing Psynergies, the Wish Psynergy series being the foremost example. Not only are the Psynergies easier to perform, reliably strong, and oftentimes distributed on multiple Adepts because of how many classes they are on, they do not put a Djinni on standby and potentially lower the user's class. This generally means that Surge distinguishes itself from other group healing effects by being a group heal that also partially sets up for a Mercury summon, so this is probably the best use of the Djinni in battle.
Golden Sun: Dark Dawn: Surge is found early in the game, but interestingly enough its Venus equivalent in this game, Flower, is found even earlier, at Konpa Gate. These Djinn may be used as minor group heals that set up for summons, but like in The Lost Age, they have a superior 50%-healing counterpart that basically obsoletes them, the Mercury Djinni Coral, though this Djinni can be found roughly in the middle of the game, at Port Rago. Flower is present for the boss battle against the dangerous trio of Stealthy Scouts in Konpa Cave, whereas Surge does not have that distinction. On the other hand, it could be said that it is more "appropriate" for a Mercury Adept used to heal and support the party to unleash a group-healing Mercury Djinni like Surge, than for a damage-dealing Venus Adept to unleash a group-healing Venus Djinni like Flower.