I ran into "dead magic" when I tried to cast a healing spell, but I don't think I've heard of it before. Anyone who could enlighten me?

Edit: I can tell, that I was slowed at the moment, perhaps thats the reason?

Dead magic zones are places where magic simply doesn't work. The only way to "fix" it is to leave the area. (Luckily, the only ones in the game are in Watcher's Keep, so you won't run into it very often.)

You are in watchers keep on the 4th level? There are certain rooms in there where magic won't work and others that will cause a wild surge affect on all magic. Just move to a new room in the maze and you should be alright.

Trouble is, I was fighting TorGal in the D'arnise keep when the text message said "Dead magic". I was trying to heal myself and the magic fuzzled... I've been there several times before but never encountered this. I've the improved TorGal mod, but again I've confronted him before with this mod installed and never seen this before. Could it stem from the slow spell?

Not because of the Slow spell. Perhaps Improved Tor'Gal introduces wild/dead magic into the encounter somehow, or perhaps you're a Wild Mage and had an unlucky surge? I think 'Dead Magic' is one of the possible outcomes.

Well I killed Improved Torgal twice but never encountered a dead magic area or trick. You are not a wild mage eh, as you were casting a healing spell? I do not know the cause. Are you playing on insane? Maybe on insane Torgal has a dead magic aura or such? Or is it a bug? Does it happen just near Torgal or everywhere now? I am confused!