Lingering Spell

Cost: a number of sorcery points equal to the spell's level

When you cast a damaging spell with an instantaneous duration that requires a saving throw, you can spend a number of sorcery points equal to the spell’s level to prolong the spell’s damage. A creature that failed its saving throw against your original casting of this spell must succeed on the same saving throw at the start of your next turn or take half of the spell’s damage.