Spirit Tortoise

The Spirit Turtle is a mountain sized turtle known to be a greater disaster than a wave of calamity.

History
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 on how to kill it, and also left warnings about why they chose to seal it instead of killing it in their time, which are presently unreadable.

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 invidually. 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 and Itsuki's teammates to abandone 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, is left to find a way to defeat it. Allying himself with the Queen of Melromark 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 infiltrait its body.

After rejoining his allies and reaching the beast's heart, Naofumi and his group succeed in sealing it, only for it to immediately break the seal and continue its rampage, forcing them to retreat. Realising Keichi left behind hints on how to destroy the monster in Japanese, Naofumi's party and the knights assemble the ancient gliphs upon its back so that he can read them. With this, Naofumi theorises 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 Firo destroy the heart. The beast dies mere moments before it could kill anyone in Melromark's Castle Town.

It was later harvested for parts, and perishable items such as its flesh were disposed off. 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.

Skills and Abilities
The turtle's humungous weight is enough of an advantage most days, and it has a powerful gravity magic which can immobolise 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 rests 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 Hero of the Shield 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 an 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 near 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 has even been shown to produce a replacement head just moments after.

Trivia

 * As is stated in story, the beast might be based on Genbu, the Black Turtle who guards the north. Naofumi thus speculates that after the tutle there awaits at least three enemies of comparable or superior power in 3 1/2 months.
 * It is stated by Ren that after the Turtle comes Houou, likely based on the Vermillion Bird of the South, and after that comes Kirin, likely based on the White Tiger of the West, the fourth and final enemy is unknown but will likely be based on the Azure Dragon of the East if it continues to follow the pattern.