The Highcaller isn't attackable after accepting the quest to kill him.
Location: Mount of Heroes South, Sidor's End, -21449, 17262, 8546
Involved NPC: Highcaller Krish
Quest Name: The One
Repro Rate: Always
Summary: The Highcaller isn't attackable after accepting the quest to kill him.
Fixed in the new version 1.1.0.6