Do you think they're building up to bringing Magnus' father in? With the mentions of needing a powerful demon to destroy the sword combined with stuff like those promo shots of Magnus w/Clary & Jace probably seeing the vision they saw, I can see him being the one to recognize the demon they need to destroy it is his father. Also there been an emotional Magnus and Alec scene in the finale, maybe Magnus having to make some kind of terrible deal in exchange for his father's help perhaps?
i mean i would love that but actually given Clary’s gangrene-looking hand next episode, like….her hand looks very much like the demon’s hand??? so i have no idea. I will say I doubt they will bring Magnus’ father in; I don’t believe they’ll go there yet (if at all). it’s possible the demon is also a reference to Sebastian’s coming, which i think is more likely.
tho at this point I’m not speculating much on the show.