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.

Battle stats
The Serpent, which resembles a long, green, snake-like dragon, statistically has 3536 Hit Points which regenerate at varying amounts, 160 Psynergy Points, 249 Attack, 76 Defense, 135 Agility, and 46 Luck. In terms of resistance, it has a Venus resistance rating of 152, a Mercury and Mars resistance rating of 100 each, and a Jupiter resistance rating of 48, and in regards to its abilities it uses its available Venus-based attacks with a Venus power rating of 110. When felled, the Serpent yields 1995 EXP and 2898 coins and, while it isn't dropped in battle, the Cloud Brand can be picked up after Susa leaves the chamber.

What makes the Serpent unique among bosses is due partly to where it's battled: The Serpent's chamber is deep within Gaia Rock, where sunlight cannot reach...normally. By exploring Gaia Rock and solving its puzzles, anywhere from one to four beams of light can reach the Serpent, significantly weakening it. The Serpent can normally attack three times per turn, but if all four lights are lit it can only attack twice. Also, the amount of HP it recovers each turn depends on how many lights enter the chamber: If no more than one or two lights are lit, the Serpent is effectively invincible. However, most players will assume that all of the beams must be uncovered to kill the Serpent, since there is nothing to suggest doing otherwise.
 * 0 lights=2430 HP
 * 1 light=810 HP
 * 2 lights=270 HP
 * 3 lights=90 HP
 * 4 lights=30 HP

The rate at which the Serpent uses the following battle commands is dependent on whether or not all four of the lights are lit at the same time, so two use rates for each move are posted, the first being the use rate that would apply if 0, 1, 2, or 3 lights are lit, and the second being the use rate that would apply if all 4 of the lights are lit:


 * Toxic Breath: Used 53 out of 256 times or 1 out of 8 times, this monster skill has the monster generating a cloud of yellow and green gas, dealing a Venus-based attack with a power rating of 60 and range of 3 (the Adepts at either side of the specifically targeted Adept will only take 50% of the attack's force, instead of the normal 80%, however). The attack also has a chance of inflicting Poison on the affected targets.
 * Mighty Press: Used 47 out of 256 times or 1 out of 8 times, this is a Monster Skill where the monster leaps up and comes crashing down on a single Adept, letting loose a burst of purple energy on impact, dealing a Venus-based attack with a power rating of 50, with a chance of instantly downing the Adept.
 * Black Ice: Used 41 out of 256 times or 1 out of 8 times, this monster skill has the monster generating a barrage of black dust, dealing a Venus-based attack with a power rating of 80 and range of 3 (the Adepts at either side of the specifically targeted Adept will only take 50% of the attack's force, instead of the normal 80%, however).
 * Briar: Used 35 out of 256 times or 1 out of 8 times, this is a Psynergy spell that stabs the party with a series of large spiky vines, dealing a Venus-based attack with a power rating of 70 and a range of 3. It consumes 11 of the user’s PP.
 * Quake Sphere: Used 29 out of 256 times or 1 out of 8 times, this is a Psynergy spell that causes three rectangular sections of ground underneath the party jolt upward and retract into the ground quickly thrice in succession, bouncing each Adept three times, dealing a Venus-based attack with a power rating of 65 and a range of 7. It consumes 15 of its user's PP.
 * Attack: Used 28 out of 256 times or 2 out of 8 times, this is this monster's standard physical attack.
 * Cure Well: Used 23 out of 256 times or 1 out of 8 times, this is a Psynergy spell that restores roughly 150 HP to the user. It consumes 7 of its user’s PP.

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 inital 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 unwinnable if no lights are shown on the serpent. It is possible, but extremely difficult. You need to survive long enough to summon Meteor, Boreas and Thor twice to raise your own elemental strength while lowering the Serpents elemental resistance with the attack Weaken or the Djinni Sour so Judegement can finish it off with the second summon rush.

Story
Little is known about the Serpent before the events of Golden Sun: The Lost Age, but an old legend in 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 Serpent once slept peacefully within Gaia Rock (known to locals as Mt. Mikage) but, during a festival in the nearby village Izumo, the Serpent was aroused and went on a rampage. The exact cause of the Serpent's awakening is unclear; although the residents of Izumo believe their festival awoke him, it would not be unreasonable to assume that the true cause was the rain of Psynergy Stones following Mt. Aleph's eruption (the cause of many of the obstacles faced throughout the series). Since both Gaia Rock and the Serpent are aligned with the Venus element, it may also be possible that the lighting of Venus Lighthouse caused the Serpent to stir (this is also what caused Poseidon to awaken, and consequentially the tidal wave). Whatever the case may be, 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. Ironically, the sacrifice ends up being Kushinada, a close freind of Lady Uzume, the village's leader, and the fiance 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.

Cultural reference
The serpent bears a great resemblance to the Japanese Shinto serpent-dragon, Yamata no Orochi, which was defeated by the god Susano-o in order to free the princess Kushinada. The Cloud Brand found near its tail is the same as the legendary sword Kusanagi found in the Orochi's tail.