Kairos allows you to re-roll a single D6 of your choice once per turn – declare before you make the re-roll. If this is used to re-roll a single dice from a batch of 2D6, 3D6 etc, the other dice in that batch cannot be re-rolled.
Source: Warhammer Armies Project: Unofficial 9th Edition
Kairos allows you to re-roll a single D6 of your choice once per turn – declare before you make the re-roll. If this is used to re-roll a single dice from a batch of 2D6, 3D6 etc, the other dice in that batch cannot be re-rolled.