While Forrest is being frutratingly cryptic about what he knows, congratulations, SD, on a solid piece of reasoning.
Let me suggest, though, that the head may be someone special from the older Ages, turned to Myrdred's will by a binding dream. It would be more interesting, I think, if the Head were actually a principal character rather than some villager whose disembodied head was kept alive and talking by dark magics. Who the Head belonged to torments us so... :)
A member of my order insists, over and over, that the head belongs to Moagim, but I have a hard time believing that. While The Deceiver was an extremely powerful sorcerer, there's no way he was able to overcome a former Leveler when he couldn't even prevent his own capture by Soulblighter after SB learned of The Deceiver's treachery.
-David