swarm of missiles

when the spell is cast, the used crystal crashes into many shards and releases its energy creating a floating magical circle on your back. As an action you may create 2 magical missiles from this circle and choose a target for each missile. Each missile deals 1d6 + your intelligence modifier force damage. The magical circle has DC equal to your DC + 5 and on taking damage explodes in a 30ft sphere. Every creature inside the explosion radius (apart from you) must make a dexterity save against your spell save DC. your allies have advantage on that roll. on a fail, the creatures must take 20d6 force damage - 1d6 for every fired missile. On a success the creatures take half damage. When you cast this spell using a spell slot of 5th level or higher, you can create one more missile and the explosion deals extra 10d6 damage for each spell slot above 4th.