megaduke
The commander-in-chief of the Byzantine navy.
historical
Roger de Flor was given the title of megaduke and the King of Bulgaria's daughter as his wife.
Where it comes from
From Byzantine Greek μέγας δούξ (mégas doúx, literally “great leader”). By surface analysis, mega- + duke.
Adapted from the Wiktionary entry megaduke (CC BY-SA 4.0).