Wraith Construct formation?

What might be the biggest noob question in history:
In my internet travels I just read that "you can take any Wraithlord or Wraithknight in any army because you just have to take the wraith construct formation."

Okay.
Where in the hell is that formation listed? Is it current? Is it legal? Is there a catch?

If not, if I can just pop in a craftworld formation of a Wraithknight all by himself - then why is that not a more common thing?!

It IS pretty common. Several guys have used it in a GT to bring 5 wraithknights in an 1850 list. Now, most tournaments are limited to 1 gargantuan/superheavy/LoW. But still, most eldar tournament armies put a wraithknight in there.

Oh, but it's only if it's part of the craftworld warhost "decurion style" formation that wraith constructs can be taken. They're not a standalone formation, just an add-on to the warhost. So in an Eldar army using either a CAD or a Warhost, you can take a wraithknight.

There is no such thing as a Wraith Construct formation. 'Wraith Construct' is an auxiliary choice for the Eldar Warhost, Their Meta-formation or Decurion, as are 'Wraith Host' and 'Aspect Host', However the later two are also formations and thus can be taken on their own. People who try to take wraith constructs as stand alone formations do not understand 7th edition Battlebound army building rules and are cheating.