Swarm of Flying Piranhas

You summon a swarm of spiritual flying piranhas in a area of 15 feets. This spiritual creatures start to fly biting the targets inside the area. Any creature you can choose who is or begins his turn on the area must succeede in a dextery saving throw or take 6d6 of piercing damage and half of that damage if succeede. The piranhas have 30 hit points and a armor class of 13, and only can be hit by magical or silver weapons. 

At higher levels: When you cast this spell using a spell slot of 4th circle or higher the number of piranhas increase adding 5 hit points per level, and the damage in the area are up to 1d6 per level.