410.1. Stunning is the act of selecting one or more Units on the Board and rendering them Stunned.
410.1.a. Stunned is a binary state. A Unit is Stunned or it isn't.
410.1.a.1. A Stunned Unit can not be Stunned again. Example: Eclipse Herald has the ability "When you stun an enemy unit, ready me and give me +1 [M] this turn." Its controller plays a spell that reads "Stun a unit." They may choose a unit that's already stunned, but if they do, Eclipse Herald will not trigger.
410.1.a.2. Stunned Units lose the Stunned status at the beginning of the next Ending Step.
410.1.b. A Stunned Unit does not contribute its might to damage in combat.
410.1.c. A Stunned Unit must still have damage applied to it equal to, or greater than, its full might value to be killed.