The answer to your question would be that the adverb clause identified is the following one "after the artist had unveiled it" and the words it modifies are "was displayed".
An adverb clause is a group of words that function as an adverb in a sentence. Adverbs give more information about how an action was performed, that is, they answer questions like how, why, where, and when. In this case, the adverb clause gives information regarding when the artwork was displayed and it has been introduced by the subordinating conjunction "after".
