We’re in the Undermine because Xal is using Gallywix and the Goblins to repair the Dark Heart as some of the design is of Goblin origin. The reason we keep fighting proxies is that manipulation and deal-with-the-devil is Xal’s schtick. She’s powerful but her style is to (literally) be the blade in the dark or the whisper in the ear. The one who gets a person to do what she wants while making them think it’s what they want.
I predict we’ll eventually hit a moment where Xal stops using others to do her bidding and instead goes full Super Saiyan in a “If you want something done right!” moment.