Elyurias is a god that the Forest Folk worshipped.
Elyurias appears as a large wasp with a golden abdomen and rainbow colored wings and stinger.
It is unknown if Eluyrias is inherently a wasp, but it is speculated that she takes this form due to its convenience for laying her eggs, or merely appears as a wasp to humans.
Elyurias is first mentioned by Rou (the old man in the cave). After Shion shows Rou his scars and tells him his story, he asks Rou to explain what is going on and what the parasite wasps (bees) are. Rou begins by telling the story of the Forest Folk, who worshipped a Forest God, named Elyurias. The name 'Elyurias' was actually given to it by Rou, a scientist who contributed to the developement of No. 6. Elyurias is just referred to as 'she', because she lays eggs. Her gender is unclear. The Forest Folk just call her the Forest God.
The scientists/politicians of No. 6 wanted Elyurias's power so they could take ultimate control over their people, but in the end, Elyurias was the one controlling the people and their plans as revenge for destroying the Forest Folk. The mysterious deaths in No. 6 before and during the Holy Day were for Elyurias's revenge.
Elyurias was 'reborn' in Safu, when the scientist and Fennec were testing the wasp eggs to find their queen (Elyurias). The Forest God was playing with them and used Safu's body to destroy No. 6. When Safu was being held as a sample, Elyurias came and explained the situation to her. Safu agreed to allow Elyurias to use her body, and Elyurias offered to grant Safu one last wish. Safu's wish was to see the one she loved, which was Shion.
In the anime, Elyurias first appears as Safu when Shion and Nezumi make it to the control room of the Correctional Facility. At Shion's death, when Nezumi sings, Safu appears again and sings as well. Then, she changes into Elyurias as a wasp.
In the novel, on the top floor, in the mayors office in the Moondrop, Nezumi, as a Singer, sings to the rioting people crowding around the building. Elyurias, then appears in the figure of a wasp and Nezumi beckons Shion to speak. He asks if he could call her by the name of 'Elyurias', and she says that she does not mind. Shion asks for Elyurias to grant the humans one last chance, and to not give up on the humans just yet. Elyurias is skeptical, and asks Nezumi "Is that what you wish for yourself, Singer? As one of the Forest Folk, you will believe in the residents of No. 6?" Nezumi stated that the only person he believed in was Shion. He wanted to see what Shion woud build from the ruins of No. 6. Elyurias recalls the time when Nezumi was an infant and states that he had grown into an insolent man. Elyurias also tells Shion that Safu also said that she believed in him. Shion tells Elyurias that he will never forget Safu for as long as he lives. Elyurias then says she will give the people of No. 6 one last chance, and disappears after that.
The Forest Folk
Every few years or so, Elyurias would lay her eggs in human hosts. When the larvae would emerge as adults, they'd chew their way out of the humans' bodies, leaving a dead corpse behind. The Forest Folk of the Mao Forest feared this, so they had a ceremony in which they offered Elyurias a Godly Bed. The Godly Bed was a man-made host prepared from animal brains. It was an offering for implantation. Led on by a song, sang by the Singers, Elyurias would lay her eggs there. After the eggs were laid, the Godly Bed never seemed to rot or dry out; instead, it maintained an adequate level of moisture and freshness until it rotted away with the emergence of the adult wasp. It was the same way a human host aged and died immediately after an adult wasp emerged.
The Forest Folk protected the Godly Bed with their bodies and souls, as a promise with Elyurias. This pact was passed down through the ages. As long as the Forest Folk continued to protect the Godly Bed, Elyurias did not harm the people. Elyurias not only protected the people, but the forest and the land.
Then, No. 6 came along and wrenched everything from them. They burned down the settlement of the Forest Folk when they resisted; they massacred women, children, and the elderly indiscriminately. Now, the Mao Forest's former location is No. 6's airport. This was called the 'Mao Massacre'. Nezumi is a Singer, and is also the last survivor of this massacre.