Can a player play a hidden unit to a battlefield after losing combat (in response to a Deathknell trigger), and if so, what happens next?

Ruling: Rules as written allow playing a hidden unit in response to a Deathknell trigger after combat damage but before losing control of the battlefield, because contested status isn't cleared until 440.1.b (which happens after the special cleanup). However, this creates an undefined game state with no clear resolution in the current rules.

Sequence: (Most discussed interpretation)

• Combat damage occurs
• Special combat cleanup begins (322)
• All defenders are removed at step 2
• Attackers would recall at step 2 if applicable (but may not if no defenders remain)
• Deathknell trigger finalizes at step 8, going on the chain
• Priority is awarded before 440.1.b clears contested status
• Defender can play hidden unit while still controlling the contested battlefield
• Game state becomes undefined: units from both players present after combat cleanup but before contested is cleared
• Suggested ruling: Treat as a new combat showdown with same attacker/defender designations

Nuances:

• This only works if there's a trigger (like Deathknell) that creates a priority window
• Without a trigger, there's no priority window and hidden cards are lost normally
• Multiple interpretations exist: (1) run another combat, (2) treat the unit play as creating an illegal state and undo it, (3) move 440.1.b into the special cleanup to prevent the window
• The Chinese referee FAQ suggests 440.1.b occurs after all cleanups finish, potentially supporting nested combats, but this interpretation is disputed
• Most judges agree the rules are currently broken in this area and await official clarification
• For casual play, simplest ruling is to either run another combat or disallow the hidden unit play entirely