The Child is the term given to the offspring of The Entity, which came to Marble Blue a millenium ago while it wrecked havoc on the beautiful city of Paradise. It is actually a giant egg that was laid on Marble Blue. However, in order to watch over it in safety, the Incarnatus Mariv brought it on board the terraformer transport Theia and attempted to stall its hatching. Despite its weak size as compared to its parent, its strength is tremendous.
During the events of Chaos Rings III, the Child had finally hatched. In order to prevent it from trying to run loose on Theia, the remaining members of the Thousand Voyagers and Mariv tried to hold it back by placing a seal on it. However, its emanating presence caused everyone, especially non-Explorers to be critically sick. While they succeeded for a while, it eventually destroyed its seal and corrupted Mariv. Johannes arrived just in time to defeat his wife and persistently attacked the Child until the Protagonist and his team finally appeared.
The Protagonist morphed into his True Incarnatus form, where his team challenged the Child as one and finally killed it, which in turn angered the Entity as it approached Marble Blue. During the fight against the Entity, it seemingly said the words "You killed my child".