My guess would be that the magic would have been used against Morghanna. Sadly Treguard doesn't give us any real clues about its true use, but I think she'd have appeared and they'd have had a chance to spellcast at her. Since they didn't have the spell, I guess she just got straight on with the business of killing them off.

I think the magic would definitely would have been used against Morghanna, as they had been warned about her by Treguard and Merlin.  One way or the other I think they would have had to deal with her.

Hi all first post.  I thought Morghanna was probably the best female baddie of the programme (from what I remember).  Especially the time she offed Martin from the stained glass window.  Though she only made two appearances the actress who played her was kept busy, didn't she also play Velda and the Branwen?