Na Bhios Ann: Once per round, after an attack or defense roll, you may increase one of the dice you rolled by 1 and decrease one of the dice your opponent rolled by 1.
One of the techniques used to achieve "illumination," the Iombhas Forasna was used for poetic inspiration and divination, and a Druid or a Filidh could be expected to know it after passing through the eighth year of study.