If all units on a battlefield are killed during combat and there is a hidden Zhonyas gear, is the player forced to reveal and use it during the showdown, or can they wait until cleanup to move it to base?

Ruling: The player must reveal and use the hidden Zhonyas during the showdown step (before damage is applied) if they want to use it. They cannot wait until cleanup because if all units die and they lose control of the battlefield, the hidden card is trashed before they can move it to base.

Sequence:

• Showdown step begins and players are given priority
• Hidden cards (reactions) must be played during this step if the player wants to use them
• Combat damage step occurs where units deal damage
• Cleanup step occurs where hidden cards without a unit controlled by the same player are trashed
• There is no reaction timing between units dying and hidden cards being trashed

Nuances:

• Zhonyas can be activated from anywhere (not just the battlefield) and can save any unit during cleanup
• Once activated during a showdown, the Zhonyas must be used for that showdown's cleanup and cannot be saved for a future showdown in the same turn
• Players can also play cards with the "action" keyword during showdown if the chain is empty