Tonic

Tonic is a Mercury Djinni found in Golden Sun and Golden Sun:  The Lost Age.

Tonic is the sixth Mercury Djinni in the original Golden Sun, and can also be  acquired in Golden Sun: The Lost Age. Although it is not the party's sixth Mercury Djinni in The Lost Age, Tonic is still ordered as the sixth Mercury Djinni when equipped to characters. Because it is not available until after most other Mercury Djinn can be found, however, the separate Djinn list viewable with the Select button from the status  screen lists Tonic and all other Mercury Djinn from Golden Sun after all of the Djinn  introduced in The Lost Age. According to this order, Tonic is the seventeenth Mercury Djinni in The Lost Age rather than the sixth, but this order has no influence elsewhere in the game.

Basic description

 * "Heal all party ailments."

When Set, Tonic increases its Adept's base HP by 8, base PP by 3, base Defense by 2, and base Luck by 2.

When Tonic is unleashed in battle, all status ailments on each currently active party member that is not Downed are removed. This includes the conditions that would disappear at the end of each battle, including Stun, Delusion, Sleep, and Death Curse, and permanent conditions that would have to be removed manually, including Poison, Venom, and Haunt.

In Golden Sun and The  Lost Age, Tonic's unleash animation visually resembles a transparent blue image of a Mercury Djinni hovering above the party as an array of blue sparkles swivel around each Adept in order.

Locations
Golden Sun: Tonic is located in Lunpa Fortress, in a room connected to Donpa's bedroom that is inaccessible the first time you sneak into  and complete the optional dungeon. After defeating Toadonpa and letting Hammet be brought back to Kalay, you are allowed to  sneak back into Lunpa Fortress. Make your way all the way back to Donpa's bedroom again, and in a cutscene Donpa will thank you for your  help and let you enter a room to collect Tonic, which he claims is a  creature he "fought once long ago" - this seems to contradict the main  plot in that the Djinn were only released into the world for the first  time at the beginning of the game, though it is possible Donpa  encountered Tonic as soon as it was released and that it was "long"  since then in his perspective.

Golden Sun: The Lost Age: If players don't transfer data from Golden Sun to The Lost Age,  Tonic will not be among the Djinn Isaac's party has with them when they join forces with Felix's party late in the game. In this case, Tonic may be found as the "make-up" Mercury Djinni at the lone, northern house in Prox.

Analysis
General: The effect of curing all status ailments, let alone on all currently active Adepts in the battle, is exclusive to Djinn like Tonic. In one maneuver, it removes from across the entire battling party all occurrences of the status conditions that the Elixir item and Restore Psynergy remove off a single targeted ally (Sleep, Stun, Delusion, and Death Curse), the damaging status conditions that the Antidote item and Cure Poison Psynergy remove off a single targeted ally (Poison and Venom), and the Haunt status condition that can otherwise only be removed either by the afflicted ally being Downed or by paying at a Sanctum. This makes Djinn like Tonic conceptually invaluable, though there seem to be few real combat situations where such a healing effect can truly be worth it - temporary status conditions are not extremely dangerous because battles go by fast anyway and they remove themselves once the battle is over, and when anyone is poisoned during a battle, it is typical to win the battle quickly, and then use Cure Poison on each poisoned Adept in the field. The Haunt condition does not even apply itself much to be of real worry in general as well. Effects like Tonic are most useful in particular boss encounters when various status conditions are spread around a lot, particularly if you are setting up for a Mercury summon in that battle. Identical equivalents to this Djinni in Dark Dawn are the Mercury Djinni Dewdrop and the Venus Djinni Clover.

By game
Golden Sun: As Tonic is the only Djinni in this game with this particular mass-status-condition-removing effect, it is the only effect of this sort that the party has access to. There are few opportunities for Tonic to be truly useful though, because of the issues outlined above. The biggest opportunity Tonic has to be useful is in the final boss battle, where both Venom and Haunt get easily spread across the party, and summons are a good thing to set up and let loose because they deal massive amounts of damage to bosses with massive amounts of HP.

Golden Sun: The Lost Age: When Isaac's party joins Felix's party along with their returning Djinn (granted that Password data transfer is in effect), Tonic's battle ability makes it a second Salt, a Venus Djinni acquirable in Contigo as soon as that game's party enters the Great Western  Sea. The two are equally usable for whenever a particularly widespread status condition situation transpires, though in a way they are limited because those sorts of situations do not occur often; the only real beneficial use for them in this game is when multiple Adepts are afflicted with Haunt  and Death Curse in the fights against the final boss and  Dullahan.

Name Orgin
A tonic is a drink intended to restore or invigorate, hence the reason the Djinni Tonic cures status conditions.