When rolling on the Miscast Table, the Slann Mage-Priest can choose to subtract one from the result (to a minimum of 2), or add one to the result (to a maximum of 12), instead of accepting the original result.
Source: Warhammer Fantasy: 8th Edition
When rolling on the Miscast Table, the Slann Mage-Priest can choose to subtract one from the result (to a minimum of 2), or add one to the result (to a maximum of 12), instead of accepting the original result.