Mythical creatures are not only "Fire Giants."
To be more accurate, Fire Giants are just one of the "manifestations" of the myriad forms of mythical creatures.
During Lena's selection process for her "Hunting" target, Renji could follow the Succubus's perspective and see that besides Fire Giants, there were also giant ancient fish that dove into the deep sea, purple extreme birds in the thunderclouds, stone statue beasts that merged with the mountains, and shadow souls in the eternal darkness of the night in other worlds.
Although each mythical creature had a different form, their unchanging commonality was that after successfully incubating and descending into their respective worlds by occupying the "savior's" body, they would forcibly reshape the world according to their own form, making the world's creatures serve them.
