Hello hello! We haven't done any detailed plotting yet per se but my general understanding with Solar is that, over time/coming up later on, the two of them are going to work together and try to pool their powers to mutually solve the problem. Arche is going to continue to try to hide what's going on with him for a while until we get to some of those latter stages of Awful Shit Happening, while in the meantime I'm going to have him finally realizing what he's dealing with in terms of Unhalad's true nature and determining that he wants to help it somehow. Helping the restless dead move on is a thing he does in his canon so he's going to try to muddle along by himself for a while, but where El would come into it would be Arche eventually roping him in since this sort of thing seems like it'd be in El's wheelhouse. (How exactly this would happen is not planned yet lol) Then after that proceeding together, with the end goal being... getting Unhalad out of Arche (I mean, unless he can't bear to ;) ) but also just finding some kind of peace for Unhalad, or the constituent parts that make up Unhalad, however that goes.