You cannot “rename” a sub-directory in this manner, but you can use the control panel to create a subdomain, forum.mydomain.com, that points to mydomain.com/forum. You can also achieve this with a mod_rewrite rule.
How does one go about doing this? I already have content in nerfbat.com/tavern, and I’d like for tavern.nerfbat.com to be the same as the subdirectory. I want to do this so I can use a subdomain from a few of my other domains to mirror the same content (i.e. tavern.mmolexicon.com would mirror tavern.nerfbat.com) since I can’t have other sites mirror the content of a subdirectory, just a subdomain.
Do I have to “fully host” the subdomain for this to work?