Spirit Tortoise

The Spirit Tortoise (霊亀) is a mountain-sized tortoise monster, known to be a greater disaster than the Waves of Calamity. It is the first of the Guardian Beasts.

Base Form
The Spirit Tortoise is massive tortoise with a mountain and the city of Reiki on its shell. Throughout the mountain range there is occasional cracks that allow access into the interior of the Spirit Tortoise. When the beast charges it's electric beam attack, it's entire torso glows as the energy collects at the mouth.

Revived Form
After the Spirit Tortoise was improperly killed the first time, it came back in a more terrible form. This time, the Spirit Tortoise's eyes glowed red while a long web of drool dangled from its draping mouth. Whatever remained of the town on its back was destroyed and the shell was now covered in towering spike-long thorns that acts like missiles. During its fight with Fitoria, the Spirit Tortoise had grown several more heads.

Skills and Abilities
The Spirit Tortoise's enormous weight is advantageous on most days, and it has powerful gravity magic which can immobilize its foes, leaving them helpless and unable to escape being crushed under it. The beast is painfully slow, but the fact it neither eats nor sleeps makes up for this almost completely. When pushed into a corner, it is capable of firing intense lightning (electric-type magic) beam attacks from its mouth or the eyeball on its heart - this attack has only been truly blocked by the Shield Hero while using the enhanced Wrath Shield 3 but can be mostly blocked with his enhanced Soul Eater Shield as well. What's truly terrifying about this attack is that it not only wipes out mountains, but it drains all of the victims' SP, leaving them truly helpless before the next shot, which can come as soon as immediately after if it had managed to absorb enough SP in the last one. The Spirit Tortoise uses Way of the Dragon Vein magic to empower its attacks.

To compensate for its slow body and limited movement, the Tortoise is also able to summon Demon Bat familiars, as well as many others including those that resemble gorillas, yetis, floating eyeballs, caterpillar-like familiars and even zombies made from the people who lived on the city on its back - this extends to the beasts inside as well. There are several more familiars that protect the heart that acts like a biological immune system. There are Mimic Type Familiars for path concealment as well, making it hard to even find the heart. Deep within the Tortoise is the heart, which is capable of using the same strong gravity magic and high-powered beam as the head, but the latter is slightly less powerful to avoid damaging itself.

The Tortoise was rendered almost immortal due to it's incredible regenerative abilities. The only means with which to stop it ultimately is for someone to destroy the core of the Spirit Tortoise. The corridor to the core will be revealed once both the heart and the head simultaneously and then using a sealing spell to prevent it from regenerating. Beheading the Spirit Tortoise only results in the Guardian Beast to grow another head to replace it.

Background
Though the origin of the Spirit Tortoise is still unknown, it is known to be tied to both the Status Magic System (SMS) and the Waves of Calamity. The beast was sealed away by a previous group of the four Legendary Heroes, which included a man named Keichi, who left behind instructions (made illegible over time) on how to kill it. The inscription also warned readers that the Spirit Tortoise played an important role in defending against the Waves so it is better to seal the Guardian Beast than to destroy it.

The Spirit Tortoise is the first of four Guardian Beasts who are meant to protect the world from the Waves of Calamity by killing and using the souls of thousands of people to create a protective barrier. The other Guardian Beasts are the Phoenix, Kirin, and the Dragon King. They are only meant to be released if the Heroes were not strong enough to stop the Waves of Calamity by themselves. In order to fully stop the waves, they must eradicate 2/3 of all life in their world.

In order to fulfill its mission to stop the waves by collecting souls, the Spirit Tortoise created the human familiar named Ost Horai as a representative of its will. In the original unsealing procedure, she was supposed slowly and secretly collects souls. If Ost was not collecting enough souls, then the familiars would be released the collect more. Finally, if that wasn't successful, then the Spirit Tortoise's true form was to be unsealed to collect souls. Ost was supposed to give hints to the Heroes and finally show them how to unseal her true form once they had defeated her in combat. The heroes were supposed to destroy statues in underground temples which represent the three seals holding the Spirit Tortoise in place. However, this was all bypassed once Kyo improperly unsealed the Spirit Tortoise.

There are similar Protective Beasts in Glass' World which are the Chinese equivalent of the Japanese Guardian Beasts of Raphtalia's World. In Glass' World, the first Guardian Beast is the Black Turtle which was defeated. Trash #2 researched in creating copies of the Protective Beasts and merging them with people.

Volume 6
At some unknown time, Kyo had crossed over from Glass' World to Raphtalia's World with the intention to take over the Spirit Tortoise. While he was starting to unseal the Spirit Tortoise's main body, he unleashed the familiars with the intention of harvesting more familiars and collecting souls. Using their game-knowledge, Ren, Itsuki and Motoyasu were able to determine that the familiars came from the Spirit Tortoise. Knowing its location and believing they were easily powerful enough to defeat it, they were to confront it. However, as they arrived Kyo had just unsealed the Spirit Tortoise and they were immediately overwhelmed. Their parties were either killed or ran away and the three heroes were captured by Kyo. With a newly unleashed Guardian Beast at his control, Kyo used its destructive power to cause massive death in destruction. As the Spirit Tortoise rampaged in the east, it turned the souls of the dead into power which Kyo collected for his own purposes.

Eventually, the Spirit Tortoise's familiar came to Melromarc. After battling against the Spirit Tortoise's familiars, Rishia was able to determine that they originated from the Spirit Tortoise. Naofumi's party and forces from Melromarc then rushed East to fight the Guardian Beast. After a desperate battle, Naofumi's party was able to behead the Spirit Tortoise and were celebrated as heroes. With the Spirit Tortoise stopped, they investigate Reiki on the Tortoise's back and find an illegible warning from a previous hero. Despite the Spirit Tortoise being "defeated", the other three heroes remain missing and Naofumi gets a mysterious warning that the fight is not over.

While Naofumi was running to confront the Spirit Tortoise, L'Arc, Glass and Therese (who were travelling through Melromarc in secret) received a warning about the Spirit Tortoise reawakening. At the same time, L'Arc's Vassal Scythe and Glass' Vassal Fan screams in agony as the group sees Kyo with the unwilling Vassal Book. The Vassal Book tries to warn the otherworldly heroes about Kyo's plans to control the Guardian Beast. However, before they are able to stop Kyo, he teleports away. The three heroes decide to also head to the Spirit Tortoise to stop Kyo's corruption of the Guardian Beast.

Volume 7
As the main body of the Spirit Tortoise "defeated", Naofumi and his party clean up the remaining familiars and search for the other Heroes. However, a mysterious woman named Ost arrives. She identifies herself as a familiar of the Spirit Tortoise and warns Naofumi that the Spirit Tortoise in being controlled by an outside power and that fight is not over. After a strategy meeting with the Queen, they learn that the Spirit Tortoise has revived in a new, terrifying form and is heading straight for Melromarc.

Naofumi, and a defense force organized by Queen Mirellia travel out to confront the Spirit Tortoise. However, the Spirit Tortoise proves to be too powerful and regenerates any time it is injured. Just when Naofumi's party is about to be overwhelmed, Fitoria arrives as reinforcement. Fitoria decides to fight the main body while Naofumi's party and an infiltration force head inside the Spirit Tortoise. Kyo uses the Spirit Tortoise's mimic familiars to prevent Naofumi's party and the otherworldly heroes from joining forces against him.

The infiltration team attempt to destroy and seal the Spirit Tortoise's heart to prevent it from working, however the efforts prove to be useless. After finally deciphering the illegible warning left behind from the previous hero, Fitoria and the infiltration group work to destroy the Spirit Tortoise's head(s) and heart at the same time. Immediately after, they seal the heart which allows Ost to open the corridor to the Spirit Tortoise's core.

Travelling further into the Spirit Tortoise, Naofumi finds the core of the Spirit Tortoise, the other three Legendary Heroes captured, and Kyo with the Spirit Tortoise's energy. Naofumi's party is almost defeated when L'Arc, Glass, and Therese arrive just in time as reinforcements. Kyo is able to use the Spirit Tortoise's powerful energy to immobilize them all. Ost reveals herself to be the spirit of the Spirit Tortoise and unlocks a powerful shield for Naofumi. As Rishia frees the heroes and breaks the cage protecting the core, Naofumi uses his new shield to destroy the Spirit Tortoise's core - killing it for good. As he does so, the Spirit Tortoise is stopped mere moments before it could reach Melromarc's Castle Town.

Seeing that he is defeated, Kyo retreats back to his world with the Spirit Tortoise energy. As Ost fades away, she asks Naofumi to bring back the energy to save this world. Naofumi agrees. When Ost gives Naofumi special permission to travel between worlds, Naofumi, his party, and the otherworldly heroes run through the portal after Kyo.

Volume 10
Kyo uses the Spirit Tortoise's energy for his experiments to empower himself. However, he is eventually defeated and the Spirit Tortoise energy is brought back to Raphtalia's World. As a result, Raphtalia's World has three and a half months until the next wave.

The corpse of the Spirit Tortoise remained outside of Melromarc's Castle Town. It was later harvested for parts while perishable items such as its flesh were disposed of. Being too massive to actually bother moving the beast out of the way, the Queen decides it will simply be better to leave its mountainous shell where it came to lay.

Trivia

 * As is stated in the story, the beasts might be based on the Four Benevolent Animals. Naofumi thus speculates that after the Tortoise there await at least three enemies of comparable or superior power in the next wave.
 * Ren states that after the Tortoise comes the Phoenix, followed by Kirin, the fourth and final enemy is later revealed to be Dragon King. These beings constitute the 7th, 8th, 9th, and presumably 10th Catastrophe Waves respectively, with Dragon King, the 10th, being the last Wave.
 * Each Guardian Beast has a Demi-human race that closely resembles them. The Genmu, a turtle-like demi-human race, correlates to the Spirit Tortoise.
 * The Spirit Tortoise reminds Naofumi of a mountain-like dragon in another game - likely Zorah Magdaros from the game Monster Hunter.