You can use store view feature of Magento. Create custome store view and use this for custom sub-domain. See steps:
1. Create store view:
2. Create cms page and edit content as whatever you want, you can change design and layout updates also.
3. Change store view configuration
4. Create new folder in your Magento store root folder. Copy .htaccess and index.php files to this folder and change Magento run settingis:
index.php (change relative path of MAGENTO_ROOT according to sub domain folder):
.htaccess (add custom store view code to system variable)
SetEnv MAGE_RUN_CODE "sub_domain"
sub_domain is store view code which is created on step 1.
You can use this logic with any subdomain or separated domain. As a result if you enter this subdomain, your custom cms landing page will be shown with your default store theme and this page will be hidden for main store.