It seems that when the relationship between "Celica" and "Selena" reaches 4.9, equip "Succubus Ring" and sleep at home. At this point, a series of events about Selena will occur, and after the event, a black sphere will be given.
And then as time passes you can get more dark orbs from the same source. Two more once you have gotten the first one after completing all events with Selena.
