The following are any custom rule books or house rules that are not part of the standard D&D 5e ruleset.
After a melee or ranged attack, if you kill a target with damage from a critical hit, any excess damage can carry over to a creature within 5ft from that target.
Examples include an arrow that flies through a target and impales a target behind it. Or, during a flurry of strikes, your weapon cuts cleanly through one goblin and into the next, etc.