As in todays patchnotes is stated:
"Prime: Shadow Brand- Added the Assassin's Mark effect to the following skills: Prime: Fox Claw (I to III), *Prime: Shackles*"
But it doesn't provide any brand.

EDIT:
It seems like an display error. The monsters indeed becoming weaker, while fox claw or other skills are indeed create a brand.
May try repair mode. Hope that fixes it. 

hope there is an official fix because i have been waiting for a long time, i need your help with my color tunnel game