Demetius
Merlin’s maternal grandfather in Thomas Heywood’s The Life of Merlin.
Demetius’s daughter was impregnated by an incubus, Merlin’s father. Heywood has created a personal name – whether deliberately or accidentall – out of Demetia, the country which Merlin’s grandfather was said to rule.
Source
The Life of Merlin | Thomas Heywood, 1641