Meteoric Hammer

A long-handled, heavy hammer sized perfectly for a giant to throw, but functioning as a warhammer for a smaller creature. 5ft long, engraved with brass flames riveted to a dark iron head, and with a strangely ridged handle just barely large enough to hold comfortably in two hands.
This hammer is a magic weapon. When you hurl it and speak its command word, it transforms into a streak of fire, forming a line 5 feet wide that extends out from you to a target within 120 feet. Each creature in the line excluding you and the target must make a DC 13 Dexterity saving throw, taking 4d6 fire damage on a failed save, and half as much damage on a successful one. The streak of fire turns back into a hammer when it reaches the target. Make a melee weapon attack against the target. On a hit, the target takes damage from the hammer plus 4d6 fire damage. The hammer's property can't be used again until the next dawn. In the meantime, the hammer can still be used as a magic weapon.