|This spell hurls a bolt containing fiery energy; on piercing a target, it lets loose an explosion that harms nearby creatures as well.|
Explosive Bolt is a level 6 Conjurations/Fire Magic spell (but since it is only available through use of a rod of ignition, Evocations determines its damage and accuracy) which launches a piercing bolt of fire. Each target it successfully strikes explodes as though struck by a fireball, allowing you to devastate large hordes of opponents. Just be aware that it does not have perfect accuracy like a fireball, as it must strike a target first to trigger the blast, and be careful not to cast it at a target in melee range unless you don't mind burning yourself too.
Explosive Bolt was added in 0.14.