So Bayaz was trying to use that magic to destroy the Eaters, what was Glustrod trying to do when he used the magic?Here's my take on it -
All magic comes from the 'other side' where all demons were exiled to. Glustrod tried to use that raw magic and was unable to contain it therefore destroying Aulcus (I think of the residue of the magic used as radioactive, hence the barren land where nothing can grow and that any ordinary person dies suddenly when in contact with the stone thingy), Bayaz managed to contain that power by use of the work he had carried out prior to the eaters arriving.
Ferro could hear the voices as she had part demon ancestry (hence her eyes) and they wanted her to open the gateway to let them escape thier confines.
I may be wrong, but thats how I saw things.
p.s. I know a lot of this is mentioned in BTAH in a talk that Bayaz gave to them all when going to the Other Side of the World, but I dont have the book at hand and cant remember what Bayaz said!