Choose an Arcana in the Spread of a unit in range. That unit must cast that Arcana, spending Actions as necessary. You may choose any targets for the effect. If it is a continuous effect, it applies to you instead.
The unit who owns the Arcana is the one who casts the Arcana. That unit's location is used for determining range. That unit must spend any Actions necessary to cast the Arcana. The Empress will choose if "may" effects are chosen or not. If the Arcana allows other Arcana to be cast (Such as The Archer's Blitz) The Empress will choose targets for those Arcana as well. Continuous effects will go next to The Empress, and on The Empress' next turn, they will be cycled under their original owner's deck.