The Diamond boss in Second Sea is at Kingdom of Rose , specifically on Flower Hill near the Café and the Colosseum. He usually spawns under the tree on top of the hill.

Quick route

  1. Go to Kingdom of Rose in the Second Sea.
  1. Head toward Flower Hill near the Café/Colosseum area.
  1. Look for the tree at the top of the hill; that’s where Diamond appears.

Extra note

Diamond is commonly described as a level 750 boss , so he’s meant for players who are already deep into Second Sea progression.

Short version: go to Kingdom of Rose, then climb Flower Hill and check under the tree.