If I use Unchecked Power to deal 12 damage and kill all units on the battlefield, can my opponent still play a Hidden unit (like Teemo) after the cleanup?

Ruling: No, the opponent cannot play a Hidden unit after Unchecked Power resolves and kills all units. The only opportunity to play the Hidden unit as a reaction is before Unchecked Power resolves, which would make that unit a victim of the damage.

Sequence:

• Unchecked Power is played
• Opponent has opportunity to react (can play Hidden unit here, but it will take the 12 damage)
• Unchecked Power resolves, dealing 12 damage to all units
• Units die during cleanup
• No opportunity to react again until another chain begins

Nuances:

• Hidden units can be played any time you can play a reaction
• You cannot keep units safe by waiting to play them after damage resolution - they must be played before the damaging effect resolves