[Endoperez]

Shadow Blast affects 7 battle-squares. In every square, there can be up to 3 humans (but only 2 horsemen or one Jotun). So the area covered by Shadow Blast can hold up to 21 humans. Usually, the spell targeting is a bit off and some of the affected squares are empty. The spell also allows for a magic-resistance roll, so some of those affected survive the effect. AoE of 7 IS huge - but your mages' precision might not be. If you've ever tried out Immolation, you'll have noticed that only few squares of the AoE hit the enemy, because the center is at the caster and not at the enemies.

Like calmon said, a high-level Death mage makes the spell much more deadly. Both the spell's damage and AoE increase with skill, and high-level caster also makes the spell penetrate enemies' magic resistance better, and allows the caster to cast it more times before getting unconscious.