Albic's Festering Miasma

You cast fetid harm at one creature or object within range. Make a ranged attack roll against the target. On a hit, the target takes 1d4 damage of a type you choose: Acid, Cold, Fire, Lightning, or Poison. After the damage, the target of this spell must succeed a Constitution saving throw or become vulnerable to the damage type for 1 minute. It may repeat this save at the end of each of its turns. This vulnerability has no effect if a target is immune or resistant to the damage type, and existing vulnerabilities cannot be stacked. A target can only be made vulnerable to one damage type by this spell at a time, and they lose this vulnerability if you're reduced to 0 Hit Points.

At the start of each of it's turns, the target takes an additional 1d4 of the damage type they were made vulnerable to as long as they're still vulnerable to that damage type.

Cantrip Upgrade. The damage increases by 1d4 when you reach levels 5 (2d4), 11 (3d4), and 17 (4d4).