mastery vs ascendancy

mastery and ascendancy both are nouns.

Word	Noun	Adjective	Verb	Adverb
mastery	Yes	No	No	No
ascendancy	Yes	No	No	No

As nouns, ascendancy is a hypernym of mastery; that is, ascendancy is a word with a broader meaning than mastery:

mastery: power to dominate or defeat
ascendancy: the state that exists when one person or group has power over another

Other hypernyms of mastery include ascendance, ascendence, ascendency, control, dominance.

mastery (noun)	ascendancy (noun)
the act of mastering or subordinating someone	the state that exists when one person or group has power over another
great skillfulness and knowledge of some subject or activity
power to dominate or defeat