overlord vs ruler

overlord and ruler both are nouns.

Word	Noun	Adjective	Verb	Adverb
overlord	Yes	No	No	No
ruler	Yes	No	No	No

As nouns, ruler is a hypernym of overlord; that is, ruler is a word with a broader meaning than overlord:

Other hypernyms of overlord include swayer.

overlord (noun)	ruler (noun)
a person who has general authority over others	measuring stick consisting of a strip of wood or metal or plastic with a straight edge that is used for drawing straight lines and measuring lengths
	a person who rules or commands