Bart the Bard is fighting against three Black Puddings.
Bart casts Bane to protect himself from their attacks, which all three fail.
Bart then sends a lightning bolt through all three. Unfortunately, Black Puddings have the Split feature:
Split. When a pudding that is Medium or larger is subjected to lightning or slashing damage, it splits into two new puddings if it has at least 10 hit points. Each new pudding has hit points equal to half the original pudding's, rounded down. New puddings are one size smaller than the original pudding.
They split into 6 medium-sized Black Puddings.
Which (if any) of those 6 Black Puddings are affected by Bane?