Demi-humans are usually not considered the most challenging opponents in Elden Ring, but this one in particular is tricky. Swordsmaster Onze is the boss located at the end of the Belurat Gaol in the Gravesite Plain area of the Realm of Shadow, exclusively available in the mighty Shadow of the Erdtree expansion. This minor dungeon is northwest just to the left of the small lake sitting beneath the Belarut Tower.