Hey in the Kings Niche the boss won’t appear. Help
Single player? Co-op? Official PvE or PvP? Private server? Are there any builds nearby? There’s a few questions that need to be answered before any suggestions can be put forth. 
It’s a common bug.
Climb the center left pillar all way to the top. When you come down, move fast as he will have spawned in the right place and will be waiting for you.
Btw, he’s called the Tortured King. You might want to fix the tittle.