When you cast lava stone on a piece of sling ammo, the stone or bullet becomes intensely hot. As a bonus action, you can launch the heated stone with a sling: the stone increases in size and melts into a glob of lava while in flight. Make a ranged spell attack against the target. If it hits, the target takes 1d8 bludgeoning damage plus 6d6 fire damage. The target takes additional fire damage at the start of each of your next three turns, starting with 4d6, then 2d6, and then 1d6. The additional damage can be avoided if the target or an ally within 5 feet of the target scrapes off the lava. This is done by using an action to make a successful Wisdom (Medicine) check against your spellcasting save DC. The spell ends if the heated sling stone isn’t used immediately.