Your next attack gains a benefit based on its current imbue:
Elemental Arrow Imbue (Fire, Cold, Electricity, Acid, Force):
Double imbue dice (3d8 -> 6d8 for one attack)
Non-elemental Arrow Imbue (Paralyzing, Terror, Banishing, Smiting):
Deal damage based as if you were using an imbue with a base 1d4 damage, and your arrow applies its vorpal effect. This damage is Bane typed.
(With all four imbues, three of which are just elemental damage, you do a base of 7dx damage. With this effect, you'd do an extra 7d4 bane damage, and applies its vorpal effect)