Harry Potter fans - please help!

Only one question left to do in a quiz - can you help? The question is "Which subject did Gilderoy Lockhart briefly teach ?" I know he taught Defense against the Dark Arts but wonder if there is a good reason for the word BRIEFLY in the question? I believe he set up a duelling club and someone has told me that he took a class for one lesson only when another teacher was ill. No time to read the books so please can someone help me answer the question! Thanks in advance.