If my unit with Temporary dies during my Beginning Phase and triggers a Deathknell ability, can I play a unit from Deathknell to the now-empty battlefield before it becomes uncontrolled?

Ruling: Yes. When Temporary triggers during your Beginning Phase, the Deathknell ability is added to the chain as a pending item before the unit moves to the trash. Because a pending item exists on the chain, the game is in a Closed State, not an Open State. Battlefields only become uncontrolled during an Open State, so you retain control until the Deathknell resolves and you can play a unit to that battlefield.

Sequence:

• Temporary triggers at the start of your Beginning Phase, creating a chain
• Temporary resolves, issuing a Kill instruction
• Deathknell triggers and is added to the chain as a pending item
• The unit moves to the trash and cleanup occurs
• The battlefield does not become uncontrolled because the game is in a Closed State (pending item on chain)
• Deathknell resolves, allowing you to play a unit to the battlefield
• You retain control and can Hold and Score

Nuances:

• Rule 323.6 only causes battlefields to become uncontrolled during an Open State (no chain exists)
• A Neutral Closed state exists when there is no Showdown or Combat but a chain does exist
• Pending items are on the chain, preventing the Open State condition