binding of mercurial flame

silvery flame shoot out at a creature you can see making a attack roll or an saving throw, binding them in a magical everchanging flame. You roll d4 and subtract the number rolled from the attack roll or saving throw and the triggering creature suffers 1d6 fire damage immediatly. but the caster must decide before the DM says whether the roll succeeds or fails to cast.

At Higher levels. When you cast the spell using a spell slot of 2:nd or higher, the extra damage die increases by 1d6 for each slot level above 1st