htaccess - Redirect All but Sub-Directory (Ember js)

I am running my main website with Ember and because of that have my htaccess file set up to redirect all requests to my index page for routing.

RewriteEngine on

RewriteCond %{REQUEST_FILENAME} -s [OR]
RewriteCond %{REQUEST_FILENAME} -l [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^.*$ - [NC,L]
RewriteRule ^(.*) /index.html [NC,L]

However, I have since added a sub-directory called /horror that is also running on Ember. Any request made to domain.com/horror I want to have redirect to /horror/index.html again, for proper routing.

How can I configure my .htaccess file to handle this?