Delayed Blast Fireball

A beam of yellow light flashes from you, then condenses at a chosen point within range as a glowing bead for the duration. When the spell ends, the bead explodes, and each creature in a 20-foot-radius Sphere centered on that point makes a Dexterity saving throw. A creature takes Fire damage equal to the total accumulated damage on a failed save or half as much damage on a successful one.
The spell's base damage is 12d6, and the damage increases by 1d6 whenever your turn ends and the spell hasn't ended.
If a creature touches the glowing bead before the spell ends, that creature makes a Dexterity saving throw. On a failed save, the spell ends, causing the bead to explode. On a successful save, the creature can throw the bead up to 40 feet. If the thrown bead enters a creature's space or collides with a solid object, the spell ends, and the bead explodes.
When the bead explodes, flammable objects in the explosion that aren't being worn or carried start burning.

Using a Higher-Level Spell Slot. The base damage increases by 1d6 for each spell slot level above 7.