Fenrir

The Great Wolf Fenrir is an S-Rank magic beast. It perished in the Demon Wolf's Garden, where its magical influence still lingers.

Norse Mythology
In Norse Mythology, Fenrir (alternatively called Fenrisúlfr) was a wolf of calamity birthed by the trickster god Loki and a giantess, Angerboda. He is a brother of the same mother to the serpent Jormungand and underworld goddess Hel. With the knowledge Loki will only bring destruction with his bountiful strength, the gods decided to chain him. Convinced it is a test of strength by conniving gods, Fenrir breaks free of the fetters easily. In response, the gods had the dwarves forge the strongest chains ever built and presented the light-looking chains to Fenrir. Fenrir, now suspicious, only agreed to the "test of strength" a third time on the condition one of the gods placed his or her hand in his mouth. The only god brave enough to step up, knowing he or she would lose a hand, was Tyr. Sure enough, he lost his hand, and the howling Fenrir had his jaws propped open with a sword. Chains now tied to a boulder, Fenrir's drool from howling created a foamy river called Expectation. When Ragnarök arrived, Fenrir broke from his chains and devoured everything in his path. He kills the chief god Odin, whose son Vidar slays him in vengeance. Another side to the story is Fenrir may have committed no evil originally, but did after he was chained.

Biography
Fenrir is the creation of the goddess of the silver moon. By order of the Goddess, he devoured the fragments of the Dark God, and cleansed them of their. To do this, Fenrir was endowed with the ability to appropriate the powers of the opponents he devoured. In the Demon Wolf' Garden, he defeated the first fragment of the Dark God.

After completing his original task, Fenrir went in search of relatively weak sealed fragments of the Dark God, defeated them and absorbed them. In total, it absorbed four fragments.

In fact, he behaved quite in accordance with his nickname — "the Divine beast devouring the Dark God". People revered Fenrir as a representative of the goddess on earth, as a divine beast, and as God's messenger. That is why he was treated much like a God. Alas, this veneration was not destined to last forever. One day, for some unknown reason, Fenrir began to run amok and attack civilians.

Then, quite unnoticed by him, the hunger and thirst for destruction began to grow in him. Fenrir, driven by these desires, hunted for new fragments of the Dark God, in order to quickly clear them all of the taint. As a result, he could no longer resist The dark God's soul assimilating with him, and succumbed to his insatiable hunger, losing control of himself.

The Gilbard continent, already damaged by the Dark God fragment that was released from its prison, suffered even more terrible destruction from Fenrir. Many countries were destroyed, and the lives of tens of millions of people were changed forever.

However, if Fenrir was completely unable to restrain himself, the destruction would be much greater. But in reality, he didn't completely lose his mind.

The personality of Fenrir himself and the uncontrollable personality of the Dark God were constantly fighting for control of the body. Although Fenrir had reached the plain, there was no particular reason for him to go there. All he wanted was to find a place where as few people as possible lived.

Divine beasts can't die a natural death. In their bodies there is a part of a God or Goddess, so they are included in the very system of functioning of the world. That's why he was looking for a place where he wouldn't threaten the world.