Can Shen be played as a reaction to the attacker's side of the battlefield during a showdown chain, and would he be affected by Ahri's legend ability that gives attackers -1?

Ruling: Shen cannot be played to the attacker's side of the battlefield as a reaction because units can only be played to locations you control unless the card explicitly says otherwise, and attackers never control the battlefield. In a hypothetical scenario where a unit appears later as an attacker, it would receive Ahri's -1.

Nuances:

• Shen being a permanent means neither player can respond to him when played as a reaction
• Players can only respond to units if they have an enter the battlefield effect, not to the playing of the unit itself