Compressed Fire Bolt - Revamped

By casting both firebolt and control fire, 2 or more people can create a compressed firebolt that, upon impact, does the combined main spell casting ability (Charisma, Wisdom or Intelligence) plus the level of all casters to the target creature. If interrupted the spell will combust causing from 2d10 to 4d10, depending on the DC save (Dexterity), fire damage to anyone within 25 ft. 

Each caster must have control flame, at least 1 with fire-bolt.

If over 10 casters contribute the firebolt is compressed so far that it becomes a solid, but very volotile object that will deal 10d8 damage to everyone in a 100ft radius that fails a CON save under 17 when dropped.