Can a player react to a deathknell trigger that occurs after combat damage (e.g., by playing Shen or a hidden unit) before battlefield control changes?

Ruling: Yes, players can react to deathknell triggers that occur after combat damage, before battlefield control changes. The deathknell goes on the chain before contested status is removed, and the cleanup is over by that point so new chain items can be resolved.

Sequence:

• Combat damage is applied and deathknell triggers
• Deathknell goes on the chain during the Special Cleanup
• Control is still contested at this point, so the original battlefield controller maintains control
• The cleanup ends before contested status is removed
• Players can now react to the deathknell on the chain
• After the chain resolves, contested status is removed

Nuances:

• This interaction is considered problematic/broken because units will be at the battlefield when they should not be
• The attacking player who won combat won't gain control of the battlefield until after scoring, so they must wait to play units like Shen
• The defending player (Player A) playing a unit in response creates a messy situation that may not be rules-as-intended
• While the rules currently allow this reaction, it results in unclear game states