Serpent

The Serpent, referred to in retrospect as the Great Serpent in Golden Sun: Dark Dawn, is a boss in Golden Sun: The Lost Age located in Gaia Rock on the island that is the site of Izumo.

This fierce dragon, more formally known as the Great Serpent of Mikage, slept in Mount Mikage on the island of Nihan. When the Venus Lighthouse was lit, the dragon awoke. Using sunlight to weaken it, the Warriors of Vale slew the beast.

As a boss
The Serpent, which resembles a long, green, snake-like dragon, is unique among bosses partly due to where it's battled: The Serpent's chamber is deep within Gaia Rock, where sunlight cannot normally reach. By exploring Gaia Rock and solving its puzzles, anywhere from one to four beams of light can reach the Serpent, significantly weakening it. In terms of the game's code, there are five separate versions of the Serpent boss that correspond to the five separate circumstances under which its encounter can be initiated. All of them, however, are especially likely to be affected by Defense-lowering effects and share the same baseline stats:

Out of four other parameters of the Serpent that get affected depending on the number of lights shining into the Serpent's chamber by the time it is fought, its HP regeneration value is the most significant by far; this is impossibly high without any lights entering the chamber. The number of lights also dictates whether it acts three or two times per turn, whether it uses a version of its command list that has different usage chances for each of its abilities, and the level of intelligence it uses its abilities with:
 * Zero Lights: 2430 HP regenerated per turn. Acts three times per turn, uses its exclusive enemy abilities more often, and does so with.
 * One Light: 810 HP regenerated per turn. Acts three times per turn, uses its exclusive enemy abilities more often, and does so with.
 * Two Lights: 270 HP regenerated per turn. Acts three times per turn, uses its exclusive enemy abilities more often, and does so with.
 * Three Lights: 90 HP regenerated per turn. Acts three times per turn, uses its exclusive enemy abilities more often, and does so with.
 * Four Lights: 30 HP regenerated per turn. Acts twice per turn, performs basic attacks with twice as much probability as any of its other six individual abilities and generic spells, and does so with.

The command list the Serpent draws from if anything less than all four lights shine into the chamber is more difficult because its three exclusive enemy abilities will be more likely to be called, while it performs its basic Attack rarely in comparison. All versions of the Serpent that use this command set act three times per turn:

The command list the Serpent draws from while all four lights shine into the chamber is less dangerous because its basic Attack is comparatively more likely to be called and its exclusive abilities are less likely than before. Also, the Serpent only acts two times per turn when fought under that circumstance:

Battle strategy
Because the Serpent is the only boss in Golden Sun that allows the player to "choose" how much health it has, it is difficult to propose one unifying battle strategy. If the player has not weakened it with all four light beams, it will recover virtually all health after an initial summon rush the player may opt to use. The battle is essentially unwinnable in this situation. If the player did uncover all four light beams, it only recovers 30HP a turn; very unlikely to cause much trouble. This is the optimum way to defeat the Serpent.

The next step for the player is largely up to them. This boss can be dispatched through Summon rushing, but as always, the player must keep in mind that their party will be very vulnerable to all of the Serpent's Venus-based Psynergy and Monster Skills. The Serpent can also be slain through conventional battling technique, granted that all members are at least at level 20+ and with most available Djinn acquired. Sheba's Plasma may see heavy usage in this battle; even though there is only one target, Plasma is generally the strongest option aside from Jupiter Djinn, as Sheba will have no single-target Psynergy. Having Piers or any other Adepts in a class that allows access to the Cure Poison spell will be immensely helpful to the party should the status condition Poison connect when the Serpent unleashes Toxic Breath.

In both cases, players should continue casting offensive Psynergy whenever possible to keep on top of the Serpent's replenishing health meter. By this point Felix should know Revive, and because his Venus Psynergy is not very effective here, he can be designated the party's quasi-healer for this battle. Jenna and Piers should utilize their single target spells to their full potential, but a more effective method would be to use the "unleash Djinn, summon next turn" strategy. The Serpent should eventually fall.

Above, it is stated that the battle is virtually impossible to win if no lights are shown on the serpent. It is possible, but extremely difficult. The simplest way is to survive long enough to summon Meteor, Boreas and Thor twice to raise your own elemental strength, then summon them again along with Judgment to finish it off with the second summon rush, killing it in a single turn. On Hard Mode, this will not be enough; it is necessary to furthermore lower the Serpent's elemental resistance with the Psynergy Weaken or the Djinni Sour. Such tactics can also make a summon rush easier on Normal Mode.

Story
Little is known about the Serpent before the events of Golden Sun: The Lost Age, but an old legend in the village of Izumo tells of how the Great Serpent of Mikage would sleep with its body crossing the river. Where its tail fell, the Cloud Brand was hidden.

The Great Serpent once slept peacefully within Gaia Rock (known to locals as Mt. Mikage) on the island of Nihan. During a festival at Izumo nearby, however, the Serpent was roused and went on a rampage. Although the residents of Izumo believe it was their festival that awoke him, the Encyclopedia entry on the Great Serpent in Golden Sun: Dark Dawn directly attributes this occurrence to the lighting of Venus Lighthouse far away at Gondowan. The Serpent quickly made its way to the village and consumed all the food the villagers prepared for the festival.

In order to satiate the Serpent's hunger, the villagers decide to sacrifice one of their maidens to the Serpent. A lottery is held to determine who the sacrifice will be, and it ends up being Kushinada — a close friend of Lady Uzume, the village's leader, and the fiancée of Susa, Uzume's younger brother. Lady Uzume prophecies that a hero will come to vanquish the Serpent and orders barrels of Dragonsbane (Sake in the Japanese version) to be taken to the Serpent's lair. Not wishing to see Kushinada die, Susa takes it upon himself to destroy the Serpent, taking the Cloud Brand with him. Upon entering the Serpent's chamber, Susa begins feeding it barrel after barrel of Dragonsbane, gradually weakening it.

During the course of The Lost Age, Felix and his band of Adepts, as well as the scholar Kraden, finally arrive in Izumo. Initially they are only searching for unique types of Psynergy (in this case the Sand Psynergy), but, upon hearing the plight of the townspeople, they selflessly embark on their own quest to defeat the Serpent. After scaling the Elemental Rock of Earth, Felix's party come across the Dancing Idol, a peculiar animate doll that acts as a skeleton key for Gaia Rock: when placed on special altars, the Dancing Idol causes statues to move, either opening new paths or allowing light into the Serpent's lair. While Susa continues to feed the Serpent Dragonsbane, the foreign heroes flood the chamber with light. Their combined efforts weaken the Serpent enough that it can be fought outright, although it is still strong enough to deflect Susa's initial attack.

Susa recovers from the Serpent's counterattack by the time Felix's team defeat the Serpent through conventional battle. After the Serpent has been pushed to its limit, Susa again attacks the Serpent with the Cloud Brand, turning it to stone and causing its tail to melt into an odd liquid. Susa then thanks the Adepts for their help and returns to Izumo, but not before hiding the Cloud Brand at the end of the "river" of liquid, where the tip of the Serpent's tail would be (in reference to the old legend). Before Felix has a chance to follow him, a tablet appears before the Serpent. Like with the other Elemental Rocks, this tablet grants a Venus Adept, in this case Felix, a brand new Psynergy, Sand. With both their goals completed, Felix's party leaves Gaia Rock, although if the player wishes they may try out their new Psynergy to reach the Cloud Brand. Whether or not Felix takes this legendary weapon, Susa later tells him where it was hidden.

In Golden Sun: Dark Dawn, taking place 30 years later, it is mentioned that the "Dread Emperor" who had founded the Angaran nation of Sana in the interim period, Emperor Ko, ordered a military incursion against Nihan to procure the blood of the Great Serpent, which he believed would give him endless life once ingested. He would only find that the beast had long since been slain.

Cultural reference
The serpent bears a great resemblance to the Japanese Shinto serpent-dragon,, which was defeated by the god Susanoo in order to free the princess Kushinada. The Cloud Brand found near its tail is the same as the legendary sword found in the Orochi's tail.

Trivia

 * The Serpent is the only enemy in The Lost Age with a unique death cry.