If this hits a target, they must make a Constitution Saving Throw, on a successful throw they take half damage. Roll 1d4, 1d6, 1d8, 1d10, 1d12, and 1d20. The total score is the damage you deal in a damage type of the caster's choosing. The trade-off for such a potentially damaging spell is that the caster takes half of any damage dealt, rounded down (e.g. if you roll the max possible of 60, you take 30 damage. If you roll 59, you take 29).
If the damage you take would be fatal, you do not die but are instead reduced to 1hp and knocked unconscious for 1d4 turns.

At 6th level, the damage is increased to 2d4, 2d6, 1d8, 1d10, 1d12, and 1d20.
At 7th level, the damage is increased to 2d4, 2d6, 2d8, 2d10, 1d12, and 1d20.
At 8th level, the damage is increased to 2d4, 2d6, 2d8, 2d10, 2d12, and 2d20.
At 9th level, the damage is increased to 2d4, 2d6, 2d8, 2d10, 2d12, 2d20, and 1d100 (if you do not have a d100, you can use 5d20 in it's place, resulting in 2d4, 2d6, 2d8, 2d10, 2d12, 7d20 total).