Widow's Mark

You choose a creature you can see within range and magically mark it as an assassination target. Until the spell ends, the target takes additional poison damage equal to your spellcasting ability modifier whenever it suffers any form of poison damage. The target also gains disadvantage on saving throws against being poisoned. If the target drops to 0 hit points before this spell ends, you can use a bonus action on a subsequent turn of yours to mark a new creature.

At higher levels: When you cast this spell using a spell slot of 3rd or 4th level, you can maintain your Concentration on the spell for up to 8 hours. When you use a spell slot of 5th level or higher, you can maintain your Concentration on the spell for up to 24 hours.