Ao3 Mirror -

The fake site looks exactly like AO3. You log in with your username and password. Instantaneously, the scammer has your credentials. They will then use those credentials to access your real AO3 account, post spam, delete your bookmarks, or try those same login details on your email or bank account.

Understanding the legitimate infrastructure behind AO3 helps clarify why unofficial mirrors are unnecessary: ao3 mirror

Fan authors often post on AO3 specifically to avoid ads and data mining. When you use a fake mirror site, you are stripping the author of their ability to control their work (they cannot delete it from the mirror) and potentially monetizing their labor via ads. The fake site looks exactly like AO3