I'm little confused Undermine's end story

Random ethereal (Phase-Thief Azir) took Dark Heart from Gallywix.
This ethereal dont Xal’atath’s with. Reason she dont get Dark Heart.

Who this ethereal? Or we wait summer story for Midnight or Season 3?

Me feel this ethereal was from last Dragonflight . If it was Locus-Walker’s people take Dark heart?