Vaskilians dark fireball v2

When this spell is cast , dark spark of energy comes from the casters fingertip to the target location.  Upon impact the ball of necromantic energy explodes for 7d8 necrotic damage unless the target makes a dex savings throw when the damage is halved. The energy spreads around corners hitting everything in a 30 ft radius.  

Damage is increased by 1d8 per spell slot used above 4.