@Sev last it was seen wasn't he talking about letting it grow stronger inside the dark domain until he can take it to the area where he obtained his dark attribute? Because I think he wanted it to grow stronger by eating the things that pop up around there... I think he's just letting it chill inside of his dark domain until he gets around to reaching that area again...
*sips tea while watching* Im just here to watch the Chaos unfold - my pfp sauce is Yuki from "Nekomusume Shoukougun" here I'll share some good books https://mangabuddy.com/manga-list/125599