When we open the website, it tells:
`Find this special file.`
so use dirsearch(a file and dir scanner) to search for "special file" and finally we got: robots.txt and robots.php.

visit it and it says:`What is the adage of Byrgenwerth scholars?MAKE a GET request to this page with a header named 'answer' to submit your answer`
By searching google, i found the answer is `Fear the Old Blood`
Then modify your headers with `answer: Fear the Old Blood` and visit robots.php, flag came out:auctf{f3ar_z_olD3_8l0oD}