Talk:Gameplay/@comment-36603043-20190305044856/@comment-34962159-20190305071456

You need the enemy's unique item (for the Mermaid, it's the Mermaid Shell). Go to the Succubus House and find the Mermaid there (it can be tricky since all the enemies there look the same), talk to her and the Unique End is among the H-scene options.