As darkness fell, the black-clad spy stealthily scaled the city's tallest skyscraper. which word or phrase in the above sentence is an adverb?

An adverb modifies a verb. So you need to look for how something is being done. In this sentence, notice that the black-clad spy scaled the tallest skyscraper. How did he scale it? Stealthily. Therefore stealthily is the adverb of the sentence.