Beast II's True Name is Tiamat (ティアマト, Tiamato), one of the Primordial Deities in Mesopotamian Mythology/Sumerian Mythology.Before being called Tiamat, she was known as the Goddess of Beginning, the primordial sea, Nammu. Along with Abzu, she was manifested from the Sea of Dawn (黎明の海, Reimei no Umi?)—an Ocean of Void (無の海, Mu no Umi) that lies below the Underworld of Ereshkigal, said to have once been under the administration of the god Enki.
In Sumerian Mythology, the gods were given birth from the pairing of Tiamat, who is the salt water, and Abzu, who is the fresh water. Afterwards, the gods, whom are their children, waved their banner of revolution against the primeval father Abzu and acquired supremacy over the world. At this moment, Tiamat gently approved the deeds of her children. It is proof that her love for her children has won over her love for her husband.
However, the gods pointed their blades even at Tiamat, their mother. Tiamat, in her grief and disorder, gave birth to 11 Magical Beasts as her new children, and they altogether had a showdown with the gods. At the end of the conflict, Tiamat and her 11 Magical Beasts were torn apart. It is said that the gods tore up her corpse into two, forming heaven and earth from it, and thus, had established the ritual for the creation of the World of Man.
Tiamat was “a womb discarded after the Genesis”, becoming the origin of Sumerian Mythology. She was turned into a “Mother Sea” and was put to use as a breeding ground to produce life, but once the earth’s environment stabilized and after the ecosystems were established, she was banished to the World of Imaginary Numbers (虚数世界, Kyosuu Sekai) (a World on the Reverse Side (裏側の世界, Uragawa no Sekai?); an imaginary space that has no life and is not even a Parallel World) as something unnecessary. (Seeing that the ecosystem was already established, there is no need for her, one who designs life randomly, and it is said that regarding the course the lifeforms who acquired intelligence conforms to, she was nothing but an impediment by now.) Hereafter, Tiamat has been waiting in the mirror world for a chance to return back to her original world.