Delayed Damage Beam

You blast a beam of sickly green light at a target that is within 80 feet of you. The target must make a DC 20 Dexterity saving throw. If they succeed, the beam does not affect them. If they fail, the beam is absorbed into their body and they glow a pale green for one turn.
Once the target has stopped glowing, you may unleash the spell’s effects onto their body. They will take 3d10 necrotic damage. You can unleash the spell’s effects whenever you want, as long as it has not been over 3 days since you originally cast the spell.