Athaea's Detonating undead

This spell, when cast detonates one undead minion of your choice in a 10ft radial circle that you can see. The enemy must make a dexterity saving throw, against your spell dc, or else be engulfed in necromantic flame. This spell deals 3D8 necrotic damage to any creature in the radius of the effect of a failed save and half as much on a sucessfull one. 

At every second level higher than 3rd, this spell allows you to target one more undead thrall to detonate. If multipe undead hit the same target, the target only gets to make it's save once.