Unstable Thunder Bomb

You create an unstable, crackling sphere of energy at a location within 150ft of you. The sphere has a 5 foot diameter, and floats 3 feet above the ground. As long as you are concentrating on it, it will remain floating doing nothing. If at any point you end concentration, it will explode. If you look away from it, get further than 300ft away from it, or do anything that normally breaks concentration, concentration will be broken.

When it explodes, everything within 40ft of the sphere must make a strength saving throw. On a fail, they take 6d8 thunder damage, and are pushed back 30ft away from the sphere. On a success, the damage and knockback are both halfed. Huge creature take half knockback (quarter on a save success,) while gargantuan creatures take no knockback. 

The explosion is violently loud and can be heard up to a mile away. Anything within 500ft of the bomb when it explodes is deafened for 1 hour with no saving throw to save their hearing.

At Higher Levels: For each level above 5th, the damage increases by 1d8, and the radius increases by 5ft.