Barsal's Exploding Orb

You hurl a mote of lightning at a creature or object within range. Make a ranged spell attack against the target. On a hit, the target takes 1d6 lightning damage.

On a roll of 5 or 6, roll 1d8 and add it to the result.
If the d8 roll is a 7 or 8, roll 1d10 and add it to the result.
If the d10 roll is a 9 or 10, roll a 1d12 and add it to the result.

There is no limit to the number of exploding dice.  Dice also explode on critical dice.

This spell's damage increases by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 17th level (4d6).  The number of exploding dice do not scale with the base dice.  You follow the same exploding dice rules above.