Board Thread:Legitimate Canon Questions/@comment-67.82.1.39-20140303011506/@comment-108.201.174.133-20140303013240

What exactly are you confused about?

"Silverfinger says there is a 'dark Kitsune'. He says 'They call it Void or Nogitsune' he explains that they possess a host and draw power from pain and tragedy, strife and chaos.

Stiles Stilinski seems to be possessed by a Nogitsune.

Kira explains that, while Kitsune have no moral sense of good or evil, if you give offense to a Nogitsune it will react badly."

--Kitsune's Wikia Page

A nogitsune has possessed Stiles. Why? We do not know. Apparently, someone has (really) offended it.