The first mention of Liu Xinghui should be after Sirius Island came back (it should be right). In the underground of the guild, the president introduced Kildas. When Lagasas' father mentioned it again, he said it was the darkness of the devil's tail.

Based on the dialogue between Mebis and jelf after the magic show, it seems that Mebis has been fighting jelf, and the demon tail seems to be set up to overthrow jelf to a certain extent, so I think that the flowing starlight should be something that can fight jelf, and it should be the existence of the demon tail that has always advocated self-righteous ideas (Laxas's words, but I don't think this sentence should be understood like this, because the cartoon says that Mebis broke out in black magic similar to jelf after his father spoke.

At that time, I guessed that it might be a demon with powerful magic (well, in fact, a friend put forward this answer, and I analyzed it myself and agreed. )

But the answer is Mebis, which I think is a close guess, depending on how Mebis fights jelf after breaking the seal.