As you enter Nokron, the first thing you must do is to find the Ancestral Woods Site of Grace. It's on the main pathway of this place, so it's not hard to find. From the Ancestral Woods Site of ...

This boss can only be fought once you have defeated Starscourge Radahn to unlock the path to Nokron, and have lit all six lights in the Ancestral Woods area to activate the Hallowhorn Grounds boss ...