Concentration, up to 30 seconds (5 rounds).

You cause a pool of caustic poison to seep up from the ground and leech into the target. The target makes a Constitution saving throw. On a success they take 4d6 poison damage. On a failure they take damage equal to 1/16 of their hit point maximum, rounded up, and on each of your turns for the duration you may deal double the damage of the previous instance of damage from this spell (i.e. you cast it to deal 1/16 of their hit point max, reactivate to deal twice that or roughly 1/8 depending on rounding, reactivate again to deal roughly 1/4, etc.).  The spell ends if you use your action to do anything else. The spell also ends if the target leaves a 90 foot radius of you. 

If cast at 9th level, the targeted creature must make a Constitution saving throw or they die after two rounds. This effect can be removed by a Greater Restoration spell cast at 8th or 9th level, or by a Wish spell.