This is the 12156th most frequent Russian word.
императоре
"Императоре" is the prepositional or locative case form of "император," meaning "emperor."
The term 'императоре' is used in the prepositional case to indicate the topic or subject matter of the book.
Мы читали книгу о последнем римском императоре.
We read a book about the last Roman emperor.
The word 'императоре' is again in the prepositional case, as it follows the preposition 'o' (about), referencing the subject of the play.
Постановка была про французском императоре Наполеоне.
The play was about the French emperor Napoleon.
The use of 'императоре' in the prepositional case emphasizes the indirect object in the sentence, which is the emperor in the context of the legend.