Burn Out, Will-O-Wisp!

After hearing your an enemies spellcasting description, you can choose to use your reaction to attempt to interrupt a creature in the process of casting a spell. A small blue ethereal wisp is summoned and closes the distance to the target near instantaneously, where it then giggles like a small child and attempts to quench the build up of magic energies before the target releases the spell. Upon successfully quenching the energy the wisp detonates, affecting only the target.

Line of sight required, this means you MUST have a *visual line of sight to the target*, although the Will-O-Wisp can pass through objects/substances unimpeded, causing no damage.

Verbal component, this means you MUST be able to speak the spell's name verbally as an activation chant (cannot be silenced, stunned, paralysed or unconscious).

Somatic component, means you MUST be able to make a quick reactionary gesture in the target's direction. (cannot be stunned, paralysed or unconscious).

If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect and whenever a creature's spell is failed by this reaction, it takes 1D6 + your spellcasting damage modifier as magical damage of the same type as the spell being interrupted. If the spell being interrupted doesn't have a "damage type" it is instead pure, magical force damage.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the interrupted spell has no effect if its level is less than or equal to the level of the spell slot you used, and the damage to a creature who fails the save is increased by 1D6 for every spell level above 3rd.