[Tyrant]

Since it's resisted and low precision, the efficacy of Shadow Blast is widely variable. If the enemy army is so big you can't miss, or coming through the gate in a siege you're gunna nail a bunch of targets, but if they have high MR and/or Antimagic it won't do that much. In a game i'm in right now i got two mages into the HoF in a single battle by spewing SB. In another one awhile back shooting it at some MR 14 sacreds had no discernible effect.

There's a pretty short list of big AE blasto spells- Falling Frost, Falling Fire, Shadow Blast, Acid Rain, Blade Wind, Nether Darts, Magma Eruption and Astral Fire. Acid Rain is clearly superior, but rare's the mage that can do it. Among the rest it just depends on what you are fighting. Shadow Blast has some advantages- it works underwater, it does not work v undead, and it's AN damage.

Since it's unlikely that you will have many mages dedicated to casting it, those that are should always have a Spell Focus, and optionally a Void Eye if you think you can get away with it, and a Rune Smasher if you got one.