I would like a website to initially display intro.html when coming from external websites like google, or just from typing in the url. This intro.html and all internal links will link to index.php (wordpress blog so index.php can't change filename) which will then serve as the homepage. Can i do this with .htaccess, perhaps some rewritecond only on external links?
Yes, just redirect every request that does not come from your website, you can check this with HTTP_REFERER. This article should get you on the right path.