Marganus (Welsh: Margan) was a legendary king of the Britons as accounted by Geoffrey of Monmouth.
[1] He was the son of Maglaurus, Duke of Albany, and Goneril, the daughter of King Leir.
Marganus was eldest male heir of Leir and, influenced by his peers, became discontent with ruling only half of Britain.
He began a scorched earth march through Cornwall until reaching the army of Cunedagius.
[2] Marganus and Cunedagius' conflict is mentioned twice in Thomas Norton and Thomas Sackville's play Gorboduc (1561), as a warning against the possibility of civil strife: "Morgan slaine did yeld his conquered parte unto his cosins sworde in Camberland" (Act I, Scene II); "the cruell sworde bereft Morgan his life with cosyns hand" (Act III, Scene I).