It sounds like you wont be using much of the default AuthController for this as that will expect some form of username/password combo.

You can however use the Auth facade by doing whatever authentication checks you want and then calling the login() method like so:

// identify your user with your credentials (OTP)
$user= User::where('password', $password)->get();

// If you have a user, authenticate them
if ($user) {
    // Authenticate the user
    Auth::login($user);
}
This is very bare bones but you can read more here (https://laravel.com/docs/5.8/authentication#other-authentication-methods)