Pyrophobia

Each time you kill an enemy using a fire spell, you must pass a DC 11 WIS saving throw or enter a panic attack until the end of your next turn. A creature in a panic attack can not move not take any other action, including any reaction, as its mind is enveloped by a dark veil.

If you kill more than one enemy at once, for example using an AOE spell, the DC increases by 2 for each additional creature killed.