Whenever one of your Imperials causes a unit to be discarded, put 1 mastery counter on that Imperial
Each of your Imperials with 3 or more mastery counters on it gets Fortitude and "Pay 2 Force Absorb 1."
Keywords
Fortitude
Fortitude: An isolated, static ability that means, "This unit can prevent unpreventable damage to itself." Units with Fortitude can prevent damage from units with Ferocity and "damage... that can't be prevented" by using Deflect, Evade, Parry, and other damage prevention abilities. Fortitude only allows a unit to prevent unpreventable damage to itself, not to other units. Putting damage counters directly on a unit is not the same thing as doing damage, and also can't be stopped by Fortitude.
Absorb
Absorb X: An activated ability that means, "Prevent up to X damage to this unit, and it gains power for its next attack equal to the amount of damage prevented this way."