descend vs roll down

descend and roll down both are verbs.

Word	Noun	Adjective	Verb	Adverb
descend	No	No	Yes	No
roll down	No	No	Yes	No

As verbs, roll down is a hyponym of descend; that is, roll down is a word with a more specific, narrower meaning than descend:

descend (verb)	roll down (verb)
come as if by falling	gather into a huge mass and roll down a mountain, of snow
move downward and lower, but not necessarily all the way
do something that one considers to be below one's dignity
come from; be connected by a relationship of blood, for example