The phrase which is an adjective phrase in this sentence is about slavery.
Adjective phrases act as simple adjectives - they describe the noun they are referring to. So, what kind of a serial novel? About slavery. This phrase modifies the noun novel further.
In 1850 is an adverbial phrase, when Congress passed the Fugitive Slave Law is not a phrase at all but a dependent clause, and started a serial novel is a fragment, not a phrase.
