Diodra

Diodora was born some time after Ironside left Camelot. However, when Diodora was just a baby, he fell victim to a deadly disease product of the poisoning that his mother suffered during pregnancy by the poison generated by the Demon King during the new Holy War.

Although his mother died, Ironside was able to save Diodora by taking him to the new Camelot, where he was able to grow up healthily. Ironside repeatedly instructed him not to leave Camelot in order to stay healthful.

(Source: Nanatsu no Taizai Wiki)