Mark of Tzeentch

Models with the Mark of Tzeench have the Flaming Attacks and Magic Resistance (-1) special rules. In addition, if a Wizard with the Mark of Tzeentch has joined a unit with the Mark of Tzeentch that has a Unit Strength of 10 or more, they may apply a +1 modifier to any Casting roll they make. Should they leave this unit for any reason, or should the unit's Unit Strength fall below 10, this modifier is lost.