Fashionopolism v4.1 onward.
A mega menu is available on all of the above themes and allows you to show several menus from one drop down in your stores main navigation.
Open your Shopify admin and go to: themes > customize theme
On the left Sections menu open the section named: Header and scroll down until you see an area labelled: Mega Dropdown
In the field marked: Mega dropdown link title Enter the name of the link which you would like the mega dropdown to appear from. For example:
A store has the following links in their main menu:
Home | Shop | Blog | About us
Shop is entered within the Mega drop down link title field.
When the Shop link will now show a mega menu containing those 3 menus.
What you enter in that Mega dropdown link title field will decide what is shown... This must be the same name and case as a link in your main menu... Without that it will not show a mega dropdown.
Under the Content > Add menu section at the bottom you may choose up to 4 menus each of which will show when the mega dropdown menu is hovered.