When do Deathknell abilities trigger and resolve during combat cleanup, and can you play a hidden unit in response to a Deathknell trigger after your unit dies in combat?

Ruling: The rules as written (RAW) allow Deathknell triggers to be added to the chain after the combat cleanup but before the contested status is cleared in step 440.1.b, creating a priority window where players could theoretically play hidden units. However, this is widely considered unintended and broken, as it causes numerous game state issues.

Sequence:

• Combat damage is dealt in step 439.1.e
• Combat cleanup (special cleanup) occurs in step 440.1.a
• Units die during the cleanup, triggering Deathknell abilities as pending items
• After cleanup completes, Deathknell triggers finalize and go on the chain
• Players receive priority and can respond (including playing hidden units)
• Step 440.1.b clears contested status and changes control

Nuances:

• Playing a hidden unit after Deathknell but before contested status clears creates undefined behavior - the unit wouldn't be recalled, combat resolution breaks down, and it's unclear who controls the battlefield or if a new combat starts
• The retired FAQ indicated Kog'Maw's Deathknell happens after healing, supporting the interpretation that Deathknells resolve after combat cleanup
• Multiple participants argue the intended design is that contested status clearing should be part of the special cleanup itself (preventing the priority window), or that control should be lost earlier while the opponent doesn't gain control until after Deathknells resolve
• Some argue damage should trigger a normal cleanup before combat cleanup, which would make Deathknells happen even earlier and allow different interactions
• The consensus is this needs official clarification before it's properly resolved