Can you use Unchecked Power to kill all units at a battlefield, then respond to the Deathknell trigger by flipping a Hidden unit to maintain control of the battlefield?

Ruling: No, you cannot do this. When Unchecked Power resolves, there is a cleanup that kills the Deathknell unit and removes the Hidden card before the Deathknell effect triggers, because you have already lost control of the battlefield.

Sequence:

• Unchecked Power resolves
• Cleanup occurs
• Deathknell unit dies and Hidden card is sent to trash (due to loss of battlefield control)
• Deathknell effect triggers (but Hidden card is already in trash)

Nuances:

• This is a known issue that also occurs with Conquer triggers after losing control of a battlefield
• Hidden is planned to be reworked to address this type of interaction