Hellish Rebuke
The creature that damaged you is momentarily surrounded by green flames. It makes a Dexterity saving throw, taking 2d10 Fire damage on a failed save or half as much damage on a successful one.
Using a Higher-Level Spell Slot. The damage increases by 1d10 for each spell slot level above 1.
Hellish Rebuke 5e 2024 vs. 2014 edition differences
The 2024 version changes the damage type description from "fire" to "green flames" and clarifies that damage increases by 1d10 per spell slot level above 1, whereas the 2014 version specifies "damage increases by 1d10 for each slot level above 1st." The damage calculation mechanics are otherwise the same.