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.
The hero Miep Gies died at age 100. Miep helped the Frank family hide during World War II, but she is best known for finding and preserving Anne Frank's diary. She never thought of herself as a hero because she wanted ordinary people to think that they could do the right thing in the same circumstances.
