High Justice Grimstone can be found within the Ring of Law in Blackrock Depths. He can be targeted, but is not a hostile enemy. Approaching the center of the ring will start an event, and the High Justice will appear and approach one of the gates and let loose a wave of random enemies. Once these have been defeated, he will go to another gate and release a random boss monster. The boss you encounter is not related to the previous enemies in any way; they are both completely random spawns.

This NPC can be found in Blackrock Depths.