dux

Dux, ducis is third declension, masculine or feminine, a consonant stem, so the genitive plural is ducum. It is an agent noun off duco, "the one who leads", and the x of the nominative is c plus s, hiding the stem duc-.

The word is deliberately generic in classical Latin: the commander of an army, a guide on a road, the head of a faction. It is not a rank. The republican army's ranks are legatus, tribunus, imperator, and calling somebody dux says what he is doing, not what office he holds. That changes in the late empire, where dux becomes a formal military command, and by the middle ages it is a title of nobility: Italian duce, French duc, English duke. Family: duco, ductor, ducatus, dux belli. English duke, ducal, and at a distance doge.