OK, so you have
http://seo.lunarforums.com
and you want to have that show in the browser address bar, while behind the scenes you are actually going to
http://www.lunarforums.com/index.php?board=15.0?
What does your redirect look like now? Your site produces a link to http://seo.lunarforums.com... what do you do with this fictitious address when it comes in? You may have a problem if you're rewriting from a subdomain (rather than www.lunarforums.com/seo), but if the subdomain actually exists (created through cPanel), it should work. If you do a 302 redirect (not 301), it should preserve the seo.lunarforums.com URL.

My advice is to not use cPanel to do redirection, especially with subdomains and add-on domains. It doesn't handle them well. Manually write your own .htaccess rewrites.

You have to have a subdomain (seo.lunarforums.com) set up, if that's the kind of SEO you want. My advice about not using cPanel to do redirection is that you modify .htaccess yourself, not rely on cPanel to put the right code for you (it will usually break with subdomains and add-ons).

So, if you want seo.lunarforums.com to redirect invisibly to www.lunarforums.com/index.php?board=15.0, you would create an seo subdomain. In its root directory (public_html/seo/) put your .htaccess file with
This will redirect anything coming into the subdomain back to the main domain at a specific board (but still showing the subdomain in the browser). Make sure that's what you want to do.

Note that if the browser shows "www.lunarforums.com/seo", that's a separate issue. After RewriteEngine On, add

Finally, don't forget your internal URLs in your forum. For instance, in SMF there is a $boardURL defined in Settings.php, which is the URL to use for internal links. If it's "www.lunarforums.com/", because that's what you want to use for other boards in your forum, that's what visitors using your seo subdomain will see once they click off the main board's page.

You may want to consider using one of the "SEO" or "SEF" mods for a forum, such that it would be www.lunarforums.com/seo rather than using a subdomain. That might be cleaner than using subdomains. Anyway, you should explore the look and feel of several different approaches before you open your forum to the public.