Venomous Mark + Poison Soul

As a bonus action, choose one creature you can see within 30 ft. of you to cursed for 1 minute. The curse ends early if the target dies, you die, or you are incapacitated. Until the curse ends, you deal an extra 1d6 poison damage to the target whenever you hit it with a spell attack.

If the target drops to 0 hit points before the curse ends, you can use a bonus action on a subsequent turn of yours to curse a new creature. Only one creature can be affected with a venomous mark at any given time.

Poison Soul (Special)
Any poison damage you deal ignores any resistance or immunity.