If an opponent initiates a showdown at a battlefield where I have Kayn with a hidden Tide Turner, can I use the battlefield ability to send Kayn home, then reveal and swap Tide Turner with Kayn so that Kayn returns with his ability active, and if the enemy unit fails to kill Kayn it goes back to base?

Ruling: Yes, this interaction works as described. As long as the chain is empty when the opponent moves in, combat is initiated and you retain control of that battlefield until the end of that combat, even if you don't have any units there.

Sequence:

• Opponent sends a unit to initiate showdown at the battlefield
• Combat is initiated and you retain control of the battlefield
• Use battlefield ability to send Kayn home
• Reveal Tide Turner and swap it with Kayn
• Kayn returns with his ability active
• If enemy unit fails to kill Kayn, it goes back to base