What is an Underlord? Below is an example of one of the cast: Magi An Underlord is a person who has been chosen to bond with a resurrected Demigod spirit called an Eidolon.  When a person becomes an Underlord, his body goes through many changes. His skin bonds with the shell of the Eidolon that has chosen him as a vessel. Everything under the skin is converted into energy. Underlord posses the ability to resume their human form either completely or partially. When they are in human form, they become vulnerable and unable to access the full use of their powers.