Blightshot

Make an attack with a weapon that uses ammunition. The ammunition is magically coated with your blight. The attack can use your spell casting ability for the attack and damage rolls instead of using Strength or Dexterity (you must use the same modifier for both rolls).
If the attack deals damage, you can replace the attack’s damage type with Poison damage, if the target makes an ability check before the start of your next turn it takes 1d8 poison damage. 

Cantrip Upgrade. The damage increases as you reach certain levels. At 5th level, the attack deals an extra 1d8 Poison damage to the target on a hit, and the damage the target takes after making an ability check increases to 2d8. Both damage rolls increase by 1d8 at levels 11 (2d8 and 3d8) and 17 (3d8 and 4d8)