Passed, descended and arrived are the main verbs of the sentence; they are finite. Descending is a participle heading a nonfinite clause ('descending again') acting as an adverbial.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
Anonymous'We passed through a range of low arches, descended, passed on, and descending again, arrived at a deep crypt' (The Cask of Amontillado, 1846)
English 1b3am I all off here?I'm afraid so.
CalifJimPut a comma after and to see how descending again is parenthetica