Sounds like a chapter created by a fan of manga series 'The Five Star Stories'. The Mirage Knights are the elite organization of mortarhed (giant robot) pilots in the most powerful military in that setting.
If
GW is aware of the above, then I would imagine that they would prefer to just leave the chapter as a name. It makes more sense to distribute the amazing marine porn exploits to chapters that aren't likely to have people claiming "rip-off!".
If you wanted to use the name for your own chapter, then it would probably be best to look to the original source for inspiration (unless, of course, the Mirage Knights were shown in an image somewhere). The Mirage Knights' primary mortarhed is the LED Mirage, which is a pure white Mortarhed. The Mirage Knights follow the orders of Emperor Amaterasu (note the gender implicit in that title; this one's male, as opposed to the Japanese Sun Goddess that the name typically refers to), who pilots the Knight of Gold - as its name suggests, a golden mortarhed (which even in the setting is acknowledged to be ridiculously extravagent and expensive...

).
The most distinguishing feature of the LED Mirages that the Mirage Knights pilot is the rather sizeable blade mounted on the head of the mortarhed. In profile, it resembles a giant unicorn horn, albeit a blade in this case instead of a horn (and yes, it can cut things...). But the in-setting inspiration for the design is, in fact, a dragon - in this case, the LED Dragon.
On a tangential note, in the Five Star Stories setting, the Mirage Knights is composed of what are refered to as "headliners" - individuals who, due to their genetics, are incredibly fast and incredibly strong. Sound familiar?