The quest text "piercing" actually has nothing to do with what you actually need to do. 

After looking around for a while we found it - at the base of the Windmill there is a door leading inside, on the door frame there are two spaces to place the runes which will open up a secret instance for further investigation:

![Piercing indeed][1]


  [1]: https://i.sstatic.net/R0nKC.jpg