line of descent vs sept

line of descent and sept both are nouns.

Word	Noun	Adjective	Verb	Adverb
line of descent	Yes	No	No	No
sept	Yes	No	No	No

As nouns, sept is a hyponym of line of descent; that is, sept is a word with a more specific, narrower meaning than line of descent:

Other hyponyms of line of descent include family, family line, folk, kinfolk, kinsfolk, phratry, side, breed line.

line of descent (noun)	sept (noun)
the hereditary derivation of an individual	people descended from a common ancestor
the kinship relation between an individual and the individual's progenitors