Thero's Exploding Spear

You throw a spear made of lighting and it sticks to a creature or object like a sticky bomb and the caster can choose when it explodes within the 1 minute concentration. 

Make a ranged spell attack against a target, if the attack succeeds the lighting spear is stuck to it's target for 1 minute and it deals 2d4 lighting damage.  If you miss the intended target the spell fails.  During the 1 minute of concentration you can set off the detonation at any time as a bonus action after you casted it the first time dealing 6d6 of lighting damage to the target. 

Any one creature within a 10ft radius of the blast must make a dexterity saving throw on a fail they take 3d6 lighting damage. If they succeed they take half.