*idea, not a spoiler *
Invisibility - target ally can not be the target of spells. Invisibility is lost if the troop casts a spell or deals skull damage.
Could think of a troop "Master Illusionist" casting a spell that makes all allies invisible. At first it doesn't do anything, since like stealthy if all hidden they are all targetable, but once the first casts or attacks, it becomes the only targetable troop.