Spirit Tortoise

The Spirit Turtle (霊亀) is a mountain-sized turtle monster, known to be a greater disaster than the Waves of Calamity.

Skills and Abilities
The turtle's enormous weight is enough of an advantage most days, and it has powerful gravity magic which can immobilize its foes, leaving them helpless and unable to escape being trampled 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.

To compensate for its slow body and limited movement, the turtle 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 act 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 turtle 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 turtle was rendered almost immortal due to the blue hourglass within its body, the only means with which to kill it ultimately is for someone to take out both the heart and the head within moments of each other, likely before either can fully heal. Blasting its head clean off results only in the monster generating a new head just moments after.

Background
Though the origin of the turtle is still unknown, it is known to be tied to 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 (currently illegible though) on how to kill it, and also left warnings about why they chose to seal it away instead of killing it in their time.

Spirit Turtle Arc
The beast was unbound in recent times by three of the present four Legendary Heroes, Motoyasu, Itsuki, and Ren, who believed themselves capable of defeating it individually. When its rampage began, the beast and its familiars more or less ignored the Heroes, who didn't even seem to be able to phase it, and pressed on. During its early rampage, its overwhelming might caused Motoyasu's and Itsuki's teammates to abandon them, while Ren and his party fought it until only Ren was left alive.

With the other three heroes MIA, and the Seven Star Heroes a fair distance away, the Shield Hero, Naofumi Iwatani, is left to find a way to defeat it. Allying himself with the Queen of Melromarc and several other country leaders to form an allied force, Naofumi battles the turtle once to assess its power, then a second time to buy his allies the time they need to infiltrate its body.

After rejoining his allies and reaching the beast's heart, Naofumi and his group had succeeded in sealing it, only for it to immediately break the seal and continue its rampage, forcing them to retreat. Realizing Keichi left behind hints written in Japanese on how to destroy the monster, Naofumi's party and the Knights assemble the ancient glyph upon its back so that he can read them. With this, Naofumi hypothesized that the beast will only die if both the head and heart are taken out, and proceeds to destroy the head with Blutopher while Raphtalia and Filo destroy the heart. The beast then finally dies mere moments before it could kill anyone in Melromarc's Castle Town.

It was later harvested for parts, and 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 turtle there await at least three enemies of comparable or superior power in three and a half months.
 * Ren states that after the Turtle comes Houou followed by Kirin, the fourth and final enemy is later revealed to be Ouryuu. These beings constitute the 7th, 8th, 9th and presumably 10th Catastrophe Waves respectively, with Oryuu, the 10th, being the last Wave.